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A  Study 

of  the 

Magmatic  Sulfid  Ores 


BY 

C.  F.  TOLMAN,  JR. 

Associate  Professor  of  Economic  Geology 
and 

AUSTIN  F.  ROGERS 

Associate  Professor  of  Mineralogy  and  Petrography 


With  20  Plates  and  7  Text  Figures 

THE  vmiVERStTV  UBRARY 


UNiVtKSiTY  GF  CALIFORNIA 
SANTA  CRUZ 


STANFORD  UNIVERSITY,  CALIFORNIA 

PUBLISHED  BY  THE  UNIVERSITY 

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INHERITANCE  IN  SILKWORMS,  I.  Vernon  Lyman  Kellogg,  Professor 
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THE  OPISTHOBRANCHIATE  MOLLUSCA  OF  THE  BRANNER-AGASSIZ  EXPE- 
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SYNOPSIS  OF  THE  TRUE  CRABS  (BRACHYURA)  OF  MONTEREY  BAY,  CALI- 
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THE  MATZKE  MEMORIAL  VOLUME.  Papers  by  John  Ernst  Matzke,  late 
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CATALOGUE  DE  Tous  LES  LIVRES  DE  FEU  M.  CHAPELAIN.  (Bibliotheque 
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LELAND  STANFORD  JUNIOR  UNIVERSITY  PUBLICATIONS 
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WILLA  1  J,  CL/ 


A  Study 


of  the 


Magmatic  Sulfid  Ores 


BY 

C.  F.  TOLMAN,  JR. 

Associate  Professor  of  Economic  Geology 
and 

AUSTIN  F.  ROGERS 

Associate  Professor  of  Mineralogy  and  Petrography 


With  20  Plates  and  7  Text  Figures 


STANFORD  UNIVERSITY,  CALIFORNIA 

PUBLISHED  BY  THE  UNIVERSITY 

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STANFORD  UNIVERSITY 
PRESS 


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CONTENTS  PAGE 

INTRODUCTION 5 

Definition  of  magmatic  ores ;  Theories  as  to  the  segregation  of  magmatic 
ores;  Classification  of  magmatic  ores;  Deposits  studied;  Plan  of  treat- 
ment. 

PART  I.    DISCUSSION    OF    THE    BEARING    OF    MAGMATIC    DIF- 
FERENTIATION ON  ORE  SEGREGATION        ...      9 

Review  of  the  current  theories  of  magmatic  differentiation;  Theories 
favored  by  us;  Problems  to  be  investigated;  Methods  of  investigation; 
Microscopic  investigation;  Conclusions  from  our  microscopic  investiga- 
tions ;  The  evidence  on  which  the  above  conclusions  are  based ;  Replace- 
ment phenomena  in  the  magmatic  ores;  Alteration  and  later  rearrange- 
ment in  magmatic  ores;  Absence  of  pneumatolytic  and  hydrothermal 
alteration  during  ore  deposition;  Magnetite-ilmenite  magmatic  ores; 
Euhedral  magnetite  formed  at  a  late  magmatic  stage;  Other  accessory 
minerals  of  probable  late  magmatic  origin;  Stages  recognized  in  the 
formation  of  magmatic  ores. 

PART  II.    DESCRIPTION   OF  THE  VARIOUS   DEPOSITS 
OF  THE  MAGMATIC  ORES 

GROUP  I.      THE  NICKEL-  AND  COPPER-BEARING  PYRRHOTITIC  DEPOSITS 
SUDBURY,    CANADA 23 

Geology ;    Structure  of  the  ore  bodies ;   Microscopic  descriptions ;    Origin 

of  the  Sudbury  ores ;   Bibliography  of  the  Sudbury  ore  deposits. 
THE  ALEXO  MINE,  ONTARIO,  CANADA -37 

Geology;     Uglow's    conclusions;     Microscopic    description;     Summary; 

Bibliography  of  the  Alexo  ore  deposits. 
THE  FRIDAY  MINE,  SAN  DIEGO  COUNTY,  CALIFORNIA 39 

Microscopic  description;    Summary. 
THE  GOLDEN  CURRY  MINE,  MONTANA •    41 

Geology;    Microscopic  description. 
PROSPECT  HILL,  LITCHFIELD,  CONNECTICUT          ....  .43 

KNOX    COUNTY,   MAINE 44 

MOUNTAIN,  WISCONSIN      ...  .     44 

OTHER  PYRRHOTITE  DEPOSITS  IN  THE  UNITED  STATES 44 

INSIZWA  RANGE,  EAST  GRIQUALAND,  SOUTH  AFRICA  .  .     44 

NORWAY 46 

Geology;  Petrography  and  mineralogy;  Discussion  of  Vogt's  conclu- 
sions; Summary;  Bibliography  of  the  Norwegian  deposits. 


PAGE 

OTHER  EUROPEAN  OCCURRENCES 54 

Sweden;     Baden,    Horbach,    and    Todtmoos,    Germany;     Sohland    and 
Sweiderich  on  the  boundary  between  Saxony  and  Bohemia. 

GROUP   II.      THE   MAGMATIC   CHALCOPYRITE-BORNITE  DEPOSITS 

OOKIEP,  NAMAQUALAND,  SOUTH  AFRICA 56 

Geology;     Microscopic    descriptions;     Summary4,     Bibliography   of    the 

Ookiep  deposits. 
ENGELS  MINE,  PLUMAS  COUNTY,  CALIFORNIA 61 

Geology;     Microscopic    descriptions;     Summary;     Bibliography   of    the 

Engels  ore  deposit. 

REMARKS  ON  CERTAIN  OTHER  DEPOSITS  THAT  HAVE  BEEN  CLASSIFIED 

AS    MAGMATIC 

PYRITIC   DEPOSITS 65 

MAGNETITE-!LMENITE   DEPOSITS 67 

OTHER  MAGMATIC   IRON   ORES 67 

CHROMITE   DEPOSITS 68 

PART  III.    SUMMARY  AND  CONCLUSIONS 

CRITERIA  FOR  THE  RECOGNITION  OF  MAGMATIC  ORES 69 

SUMMARY  OF  THE  CHARACTERISTICS  OF  MAGMATIC  ORES 70 


EXPLANATION    OF    PLATES,    AND    METHODS    OF    PREPARING    PHOTOGRAPHS    AND 

SECTIONS 74 

PLATES,  I-XX 


ACKNOWLEDGMENTS 

This  study  has  been  made  possible  by  the  generous 
contribution  of  material  by  the  following  named  gentle- 
men: F.  L.  Hess,  Ernest  Howe,  E.  P.  Jennings,  J.  F. 
Kemp,  C.  W.  Knight,  Adolph  Knopf,  R.  D.  Longyear, 
F.  H.  Mason,  M.  E.  Morgan,  Heinrich  Ries,  A.  W. 
Rogers,  Beecher  Sterne,  H.  W.  Turner,  and  T.  L. 
Walker. 

We  also  acknowledge  our  indebtedness  to  Professor 
Bailey  Willis  for  a  critical  reading  of  the  manuscript  and 
for  valuable  suggestions. 


A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

INTRODUCTION 
DEFINITION   OF   MAGMATIC   ORES 

The  term  "magmatic  ore"  is  generally  applied  to  those  phases  of 
igneous  rocks  in  which  there  has  been  an  unusual  accumulation,  sup- 
posedly during  the  molten  stage,  of  the  accessory  ore-minerals.  The 
recognition  of  this  type  of  ore  deposits  is  due  largely  to  the  work  of 
Vogt,  who,  in  a  classic  series  of  papers,1  not  only  established  the  exist- 
ence of  magmatic  ores,  but  also  arranged  them  into  well  defined  groups, 
and  gave  the  characteristics  of  each  group.  His  classification  has  been 
followed  generally  by  the  authors  of  recent  textbooks.2  Nevertheless 
some  confusion  is  apparent  in  geological  literature  as  to  the  meaning 
of  the  term  "magmatic"  as  applied  to  ore  deposits.  This  is  due  in  part 
to  the  designation  3  of  all  deposits  of  direct  or  indirect  magmatic  origin 
as  magmatic  deposits.  For  example,  few  doubt  the  magmatic  origin  of 
contact  deposits  and  of  cassiterite-tourmaline-quartz  veins ;  but  these  are 
not  magmatic  deposits  or  segregations  in  the  strict  sense  of  the  term. 

The  term  "magmatic  deposits"  should  be  limited  to  those  segrega- 
tions of  ore-minerals  that  take  place  under  the  influence  of,  or  closely 
connected  with,  the  molten  stage  of  the  parent  rock.  Ore  accumula- 
tions accompanied  by  destructive  pneumatolytic  action,  or  those  formed 
by  hydrothermal  solutions,  are  not  to  be  classed  as  magmatic  deposits, 
altho  they  may  be  closely  related  to,  and  follow,  the  magmatic  period 

1  Vogt,  J.  H.  L. — Bildung  von  Erzlagerstatten  durch  Differentiationsprocesse 
in  basischen  Eruptivmagmata.     Zeit.  f.  prakt.   Geol.,  Jahrgang   1893,  4-ri»   I25~43» 
257-284. 

Beitrage  zur  genetischen  Classification  der  durch  magmatische  Differ- 

entiation-processe  und  der  durch  Pneumatolyse  entstanden  Erzvorkommen.  Zeit. 
f.  prakt.  Geol.,  Jahrg.  1894,  381-399. 

Weitere  Untersuchungen  uber  die  Ausscheidungen  von  Titaneisenerzen  in 

basischen  Eruptivgesteine.  Zeit.  f.  prakt.  Geol.,  Jahrg.  1900,  233-242,  3/0-382; 
Jahrg.  1901,  9-19,  180-186,  289-296,  327-340. 

2  As  is  noted  below,  Vogt  classifies  as  magmatic  certain  pyritic  deposits  that 
we  believe  should  not  be  included  in  the  magmatic  group,  and  also  omits  one  of 
the  most  important  divisions  of  this  group,  viz.,  the  chalcopyrite-bornite  type  of 
magmatic  ores. 

3  Geijer,  Per. — Iron-ore  geology  in   Sweden  and  America.     Econ.   Geol.,   10, 
231   (1915). 


6  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

of  ore  concentration.  In  as  much  as  ore  concentrations  connected  with 
persilicic  ("acid")  rocks  are  of  the  latter  type,  the  typical  magmatic 
deposits  are  confined  to  the  subsilicic  ("basic")  rocks. 

THEORIES  AS  TO  THE  SEGREGATION  OF  MAGMATIC  ORES 
Altho  the  chief  types  of  magmatic  ores  are  well  recognized,  there 
is  considerable  difference  of  opinion  as  to  the  details  of  the  processes 
by  which  the  ores  are  formed,  and  as  to  the  "order  of  crystallization" 
of  the  ore-minerals  with  reference  to  the  silicates.  These  different 
ideas  may  be  grouped  broadly  as  follows : 

1.  Magmatic  ores  have  been  defined  as  unusual  accumulations  of 
certain  of  the   accessory  minerals   of   the   igneous   rocks;  and  as   the 
accessory  minerals  are  generally  believed  to  be  the  first  to  crystallize 
out  of  the  magma,  according  to  the  order  of  crystallization  suggested 
by  Rosenbusch,  the  natural  inference  is  that  the  ores  are  the  first  to 
form,  and  that  they  settle  by  gravity  to  the  base  of  the  magma.     For 
example,  it  is  generally  assumed  that  the  segregation  of  the  large  bodies 
of  iron  ore  in  the  Bushveldt  laccolith  in  South  Africa  and  in  the  Duluth 
laccolith  and  the  sulfid  masses  in  the  Sudbury  laccolith  have  been  con- 
trolled by  gravity.4 

Beyschlag,  Krusch,  and  Vogt 5  state  "Bei  den  meisten  Eruptivge- 
steinen  beginnt  die  Kristallisation  mit  der  Aussonderung  der  sogenannten 
Erzmineralien,  wie  Magnetit  oder  Titanomagnetit,  Eisenglanz,  Ilmenit, 
Zirkon,  Apatit,  Schwefelkies,  mitunter  auch  Spinel  u.  s.  w.  In  einer 
etwas  spateren  Stufe  der  Verfestigung  beginnen  die  Eisenmagnesium- 
silikate,  wie  Glimmer,  Hornblend,  und  Pyroxenmineralien  und  Olivin, 
zu  kristallisieren." 

2.  Vogt,  who  has  examined  the  magmatic  ores  both  in  the  field 
and  with  the  microscope,  favors  the  hypothesis  that  prior  to  the  crys- 
tallization of  the  rock-minerals  the  ores  separate  as  an  immiscible  sulfid 
or  oxid  melt,  which  continues  in  the  molten  state  during  the  consolida- 
tion of  the  silicates,  intrudes  the  latter,  and  finally  crystallizes. 

3.  Modern  investigation  of  polished  surfaces  of  ores  has  led  to  the 
discovery  that  the  magmatic  sulfid  ore-minerals  have  a  definite  order  of 
crystallization,  and  that  the  younger  minerals  intrude  and  replace  the 
older.    To  meet  this,  Howe  has  framed  a  modification  of  Vogt's  hypothe- 
sis.    He  suggests6  that  the  different  minerals  which  make  up  the  sulfid 


4  Daly,  R.  A. — Igneous  rocks  and  their  origin,  454  (1914). 

5  Die  Erzlagerstatten  der  Nutzbaren  Mineralien  und  Gesteine,  1,  240  (1910). 

6  Howe,    E. — Petrographical   notes    on    the    Sudbury   nickel    deposits.      Econ. 
Geol.,  9,  522  (1914). 


INTRODUCTION  7 

matte  are  mutually  immiscible,  and  the  sulfids  last  to  crystallize  intrude 
the  previously  solidified  sulfids. 

4.  Our  study  of  the  magmatic  ores  has  led  us  to  frame  the  hypothe- 
sis that  the  magmatic  ores  in  general  have  been  introduced  at  a  late 
magmatic  stage  as  a  result  of  mineralizers,  and  that  the  ore-minerals 
replace  the  silicates.  This  replacement,  however,  differs  from  that  caused 
by  destructive  pneumatolytic  or  hydrothermal  processes  in  that  quartz  and 
secondary  silicates  are  not  formed  at  the  time  the  ores  are  deposited. 

None  of  the  magmatic  ores  are  entirely  free  from  alteration,  and 
in  some  cases  they  are  associated  with  high-temperature  alteration 
products  as  well  as  with  those  of  hydrothermal  origin.  It  became 
necessary  for  us,  therefore,  to  study  the  various  stages  of  mineral  deposi- 
tion, especially  the  migrations  and  alterations  that  follow  the  original 
deposition  of  the  ore. 

Some  of  the  data  indicating  that  the  magmatic  ores  are  later  than 
the  silicates  have  influenced  a  number  of  geologists  to  question  the  pro- 
priety of  classifying  certain  deposits  as  magmatic  and  perhaps  to  doubt 
the  importance  of  the  type  as  a  whole.  Our  work  meets  the  objections 
urged  by  opponents  of  the  magmatic  theories  in  that  it  proves  the  ores 
are  formed  within  the  magmatic  period,  as  defined  by  us,  altho  the  ore- 
minerals  are  later  than  the  silicates. 

CLASSIFICATION  OF  THE  MAGMATIC  ORES 

Vogt7  separates  the  magmatic  ores  according  to  mineral  composi- 
tion into  three  groups,  viz. :  the  oxid  ores,  the  sulfid  ores,  and  those  of 
the  native  metals.  The  last  group  includes  nickeliferous  iron,  plati- 
num, copper,  and  gold.  The  magmatic  deposits  of  nickeliferous  iron 
and  of  platinum  are  curiosities  rather  than  ore  deposits.  The  existence 
of  magmatic  copper  and  gold  has  been  asserted,  but  not  proved  by 
accurate  microscopic  work.  Therefore  they  are  not  considered  further 
by  us.  The  oxid  group  includes  deposits  of  chromite,  corundum, 
and,  most  important,  of  magnetite  and  ilmenite.  Altho  we  touch  upon 
these  briefly,  the  study  presented  here  deals  chiefly  with  the  magmatic 
sulfid  deposits.  These  sulfid  ores  have  been  classified  as  magmatic 
because  of  their  exclusive  occurrence  in  igneous  rocks;  because 
the  ore  formation  is  not  accompanied  by  pneumatolytic,  contact,  or 
hydrothermal  silicates,  so  markedly  developed  in  connection  with  sulfid 
deposits  of  the  non-magmatic  types  in  igneous  rocks ;  and  because  the 


7  Zeit.  f.  prakt.  Geol.,  Jahrgang  1894,  p.  382. 


8  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

ore-minerals  have  been  considered  by  some  to  be  contemporaneous  with, 
or  even  earlier  than,  the  silicate  minerals  of  the  igneous  rock. 

The  magmatic  sulfid  ores  have  been  divided  into  three  groups: 
(i)  the  pyrrhotite-chalcopyrite  deposits  in  norite  and  gabbro;  (2) 
chalcopyrite-bornite  deposits  in  norite  and  diorite;8  and  (3)  the  so- 
called  intrusive  pyritic  ores.  We  mention  the  third  group  but  briefly, 
and  doubt  the  propriety  of  classifying  these  deposits  either  as  "intrusive" 
or  as  "magmatic." 

DEPOSITS  STUDIED 

Of  the  first  group,  the  examples  studied  in  detail  by  us  include  the 
ore  deposits  at  Sudbury,  Canada;  those  of  the  Alexo  mine,  Canada; 
and  of  minor  importance,  the  deposits  at  Litchfield,  Conn. ;  in  San  Diego 
county,  California,  and  of  the  Golden  Curry  mine,  Montana.  We  also 
summarize  the  important  data  available  in  the  literature  on  the  deposits  of 
Norway,  Sweden,  Saxony  and  Bohemia,  and  South  Africa.  The  de- 
posits of  the  second  group  studied  include  those  of  Ookiep,  Namaqua- 
land,  South  Africa,  and  of  the  Engels  mine,  Plumas  county,  California. 

PLAN  OF  TREATMENT 

In  the  first  portion  of  this  article  we  discuss  briefly  the  conclu- 
sions in  regard  to  magmatic  differentiation  in  general  that  we  believe 
are  justified  by  recent  studies  by  Bowen  and  others,  and  summarize  our 
conclusions  regarding  the  magmatic  ores,  without  referring  in  detail 
to  the  microscopic  and  field  data  on  which  they  are  founded. 

In  the  second  part  we  present  our  studies  of  the  various  sulfid 
ores  examined,  and  summarize  the  critical  data  found  in  the  literature 
in  regard  to  each  occurrence. 

Part  three  contains  a  brief  summary  of  our  conclusions  and  a 
statement  of  the  criteria  by  which  the  magmatic  ores  may  be  recog- 
nized. 


8  The  second  subdivision  of  the  magmatic  sulfid  ores  was  suggested  by  Stutzer 
[Zeit.  f.  prakt.  Geol.,  15,  311  (1907)]  and  the  importance  of  the  group  was  estab- 
lished by  the  detailed  descriptions  of  the  productive  deposits  in  Little  Namaqua- 
land  [Rogers,  A.  W. :  The  nature  of  the  copper  deposits  of  little  Namaqualand. 
Proceed.  Geol.  Soc.  of  South  Africa,  Jan.  31,  1916],  and  of  the  Engels  mine, 
California  [Turner  and  Rogers  (A.  F.)  :  A  geologic  and  microscopic  study  of  a 
magmatic  copper  sulfid  deposit  in  Plumas  county,  California,  and  its  modification 
by  ascending  secondary  enrichment.  Econ.  Geol.,  9,  359-391  (1914)]. 


PART  I. 

DISCUSSION  OF  THE  BEARING  OF  MAGMATIC 
DIFFERENTIATION  ON  ORE  SEGREGATION 


REVIEW  OF  THE  CURRENT  THEORIES  OF  MAGMATIC  DIFFERENTIATION 

Magmatic  deposits  mark  the  beginning  of  the  ore- forming  pro- 
cesses, and  they  register  the  early  stages  of  ore  formation,  often 
little  modified  by  the  complicated  processes  that  follow.  The  science 
of  ore  deposits  is  largely  theoretical  on  account  of  the  lack  of  definite 
information  regarding  the  relation  of  the  ores  to  the  associated  minerals. 
Our  microscopic  study,  carried  on  during  the  past  two  years,  is  an 
attempt  to  get  at  the  facts  for  this  class  of  ores — a  class  generally  ne- 
glected by  American  geologists. 

The  conclusions  we  have  reached  in  regard  to  the  origin  of  the 
ores,  and  to  the  order  of  sequence  of  the  ore-minerals,  appear  to  us 
to  have  a  broader  application  than  to  nature's  processes  of  igneous 
metallurgy.  Altho  magmatic  ore  segregations  are  merely  uncommon 
rock  types,9  nevertheless  their  study  reveals  certain  of  the  magmatic 
processes  more  clearly  and  more  in  detail  than  do  the  simpler  non-ore- 
bearing  rocks.  The  relatively  new  microscopic  study  of  ores  may, 
perchance,  make  some  contribution  to  the  older  science  of  microscopic 
petrography. 

The  present  time  is  especially  opportune  for  the  presentation  of 
our  results  because  the  recent  work  of  Bowen,10  along  both  theoretical 
and  experimental  lines,  affords  a  foundation  in  the  definite  conclu- 
sions reached  in  regard  to  the  important  factors  governing  rock 
differentiation,  and  his  work  possibly  has  relegated  to  the  scrap  heap 
of  discarded  theories  a  number  of  the  hypotheses  as  to  the  processes 
of  differentiation.11 

In  view  of  the  detailed  discussion  by  Bowen  it  is  only  necessary 
to  mention  the  two  important  groups  of  hypotheses  that  he  now  con- 

9  Crook,  T. — The  genetic  classification  of  rocks  and  ore  deposits.     Mineralog. 
Mag,  17,  55  (1914). 

10  Bowen,   N.   L. — The   later   stages   of  the  evolution   of  the   igneous   rocks. 
Jour.  Geol.,  23   (supplement),  1-91  (1915). 

11  For   an   excellent    summary   of  the   theories   of   differentiation   see   L.   V. 
Pirsson,  Bull.  U.  S.  Geol.  Surv,  no.  237,  183-189  (1906). 


10  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

siders  untenable,  in  order  to  clear  the  discussion  of  the  conclusions  that 
are  based  on  them.  The  first  group  includes  those  hypotheses  favored 
by  Vogt,  Iddings,  and  Becker  that  postulate  the  segregation  of  mineral 
compounds  in  a  molten  magma  by  diffusion  in  a  single  liquid  phase. 
The  flow  or  diffusion  of  the  materials  first  to  crystallize  may  be  towards 
the  cooler  portion  of  the  magma  reservoir  (the  sides)  according  to 
the  Soret  principle,  or  the  segregation  may  be  assisted  by  convection 
currents  in  the  magma  (Becker),  or  may  be  controlled  by  gravity 
("density  stratification").  The  second  group  includes  the  so-called 
liquation  hypotheses,  which  postulate  the  formation  of  immiscible  liquid 
phases  (Rosenbusch,  Backstrom,  and  others).  This  group  of  theories 
has  been  favored  by  geologists  because  it  affords  a  ready  explanation  of 
many  of  the  observed  field  relations,  such  as  the  successive  intrusion 
of  "basic"  and  "acid"  dikes,  etc.  However,  no  evidence  of  even  minute 
globules  indicating  immiscible  liquids  has  been  found  in  lavas  or  in  the 
quenching  experiments12  carried  on  in  the  Geophysical  Laboratory  at 
Washington. 

THEORIES  FAVORED  BY  Us 

We  believe  that  differentiation  is  accompanied  chiefly  by  sinking 
of  crystals  of  an  early  generation  to  form  "basic"  rocks,  as  emphasized 
by  Bowen,  and  the  resultant  segregation  of  "acid  extracts"  and  gases 
in  the  liquid  portion.13 

We  emphasize  the  hypothesis  that  differentiation  involves  two 
distinct  and  at  the  same  time  complementary  processes :  ( I )  Initial  differ- 
entiation takes  place  at  an  early  stage  in  the  consolidation  of  the  magma, 
at  high  temperatures,  and  under  relatively  anhydrous  conditions, 
and  results  in  the  concentration  of  certain  ferromagnesian  minerals  by 
the  sinking  of  the  early- formed  crystals.  (2)  As  a  result  of  the  crystal- 
lization and  removal  of  the  mafic  minerals,  there  is  a  concen- 
tration in  the  still  fluid  magma  of  the  felsic  constituents,  of 
gases  and  of  mineralizers,  of  those  elements  (chiefly  the  precious  and 
base  metals)  the  crystallization  of  which  is  delayed  by  mineralizers 
(chlorin,  fluorin,  boron,  water,  hydrogen  sulfid,  etc.).  This  process 
finally  develops  a  series  of  "acid  extracts"  which  form  pegmatite  and 
aplite  dikes,  contact  deposits  (by  the  reaction  of  these  extracts  upon 


12  Bowen,  N.  L.— Loc.  cit.,  9-10. 

18  Smyth,  C  H.  Jr. — The  chemical  composition  of  the  alkaline  rocks  and  its 
significance  as  to  their  origin.  Am.  Jour.  Sci.,  (4),  36,  33-46  (1913). 

Lane,  A.  C. — Wet  and  dry  differentiation  of  igneous  rocks.  Tufts  College 
Studies  (Scientific  series),  3,  39-54  (1910). 


MAGMATIC  DIFFERENTIATION  AND  ORE  SEGREGATION  11 

calcareous  rocks),  high-temperature  quartz  veins  of  the  pneumatolytic 
and  allied  types,  and  the  various  succeeding  families  of  intermediate- 
temperature  ore  deposits.14 

Owing  to  the  vicissitudes  of  successive  differentiation  and  intru- 
sion, and  possibly  of  remelting  and  reintrusion,  individual  masses  of 
"basic"  composition  are  encountered,  such  as  peridotites,  gabbros, 
diorites,  and  others  of  "acid"  composition  such  as  quartz  mon- 
zonites,  quartz  diorites,  granites,  etc.  We  believe  that,  on  cooling,  each 
must  have  undergone  an  early  ("basic")  and  a  late  ("acid")  differ- 
entiation. In  the  subsilicic  rocks  one  would  expect  the  products  of  initial 
differentiation  to  be  most  important,  and  the  final  "acid  extracts"  to 
be  small  in  amount  and  feeble  in  action;  while  the  products  of  the  first 
crystallization  of  persilicic  magmas  should  not,  in  general,  be  rich  in  the 
mafic  silicates  and  oxids,  but  the  final  extract  should  develop  abundant 
pegmatite  dikes  and  quartz  veins.  The  latter,  altho  often  important 
as  ore  carriers,  are  not  considered  magmatic  ores  in  the  strict  sense 
of  the  term,  and  for  this  reason  we  confine  our  attention  to  the  sulfid 
and  oxid  ores  segregated  in,  and  at  the  margins  of,  gabbro  and  norite 
intrusions. 

PROBLEMS  TO  BE  INVESTIGATED 

Our  task  is  to  determine  whether  these  ores,  chiefly  magnetite, 
pyrrhotite,  chalcopyrite,  and  bornite,  are  the  early-formed  minerals 
of  the  magma,  or  whether  they  are  formed  at  a  late  stage,  and  are 
accompanied  and  segregated  by  the  action  of  an  "acid  extract"  devel- 
oped by  the  crystallization  of  a  dominantly  "basic"  rock.  The  question 
is  of  importance  in  the  general  theory  of  ore  deposits.  Is  ore  forma- 
tion connected  with  the  gaseous  extracts  developed  during  the 
late  stages  of  the  consolidation  of  igneous  rocks?  Or  are  there,  on  the 
other  hand,  two  unrelated  processes  of  ore  formation,  one  confined  to 
the  early  stage  of  the  consolidation  of  the  magma,  and  only  of  import- 
ance in  femic  rocks,  and  the  other  to  a  later  stage,  and  developed 
chiefly  in  connection  with  salic  rocks?  If  so,  the  two  types  of  ore  will 
be  concentrated  at  different  places,  one  at  the  base  of  the  intrusive 
magma  and  the  other  near  its  upper  and  outer  margins.  Their 
accumulation  will  be  effected  by  different  factors.  The  correct 
answer  to  the  question  may  throw  light  on  the  obscure  problems  of 
the  crystallization  of  deep-seated  magmas.  Do  all  the  minerals  of 
igneous  rocks  crystallize  in  a  regular  order  according  to  the  laws 


i*Tolman,  C.  F.— The  magmatic  origin  of  ore- forming  solutions.     Min.  and 
Sci.  Press,  104,  401-404  (1912). 


12  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

of  solubility  in  anhydrous  melts,  or  are  the  relations  more  compli- 
cated? Do  the  gaseous  extracts  react  on  the  earlier  minerals  to  form 
new  minerals ;  and  is  there  in  the  later  stages  actual  replacement  of  one 
substance  by  another?  Are  these  stages  followed  by  others  that  are 
typically  hydrothermal  ?  Can  these  various  stages  be  recognized  and 
distinguished  from  each  other? 

METHODS  OF  INVESTIGATION 

These  problems  may  be  attacked  by  three  different  methods  of 
research,  viz.:  (i)  experimental  investigations;  (2)  field  investigations; 
(3)  microscopic  investigations. 

Experimental  Investigations. — Simple  rocks  consisting  of  not  more 
than  four  components  can  be  made  and  investigated  in  the  laboratory. 
As  yet  these  have  been  studied  only  under  anhydrous  conditions  and 
without  the  addition  of  volatile  mineralizers.  The  work  of  the  staff 
of  the  Geophysical  Laboratory  at  Washington  has  added  to  the  accuracy 
of  our  knowledge  of  crystallization  under  these  conditions.  How- 
ever, both  Day  15  and  Bowen  16  recognize  the  importance  of  the  action 
of  mineralizers,  but  as  yet  the  difficult  problem  of  studying  crystalliza- 
tion under  the  control  of  volatile  mineralizers  has  not  been  undertaken. 
Bowen  17  states :  "It  will  probably  be  a  long  time  before  important  aid 
in  attacking  the  questions  can  be  expected  from  the  experimental  side, 
on  account  of  the  difficulty  of  treating  systems  containing  volatile  com- 
pounds." Until  these  are  investigated  we  have  not  advanced  far  in  the 
solution,  along  experimental  lines,  of  the  fundamental  problems  of  rock 
and  ore  genesis.  Up  to  date  experiment  has  taught  us  that  the  common 
minerals  such  as  olivine,  the  pyroxenes,  and  the  lime-soda  feldspars 
can  segregate  by  sinking  of  these  crystals  in  a  melt.  Analogy  sug- 
gests, perhaps,  that  the  accessory  minerals,  especially  magnetite  and 
sulfids,  which  are  commonly  thought  to  be  among  the  earliest  of  the 
minerals  to  crystallize  out  of  a  magma,  might  segregate  in  a  similar 
manner.  However,  magnetite,  apatite,  ilmenite,  and  the  sulfids  are  car- 
ried on  by  mineralizers  into  the  stages  of  the  formation  of  pegmatites 
and  quartz  veins,  and  therefore  are  formed  at  a  late  stage  of  rock  con- 
solidation. The  problem,  then,  as  to  whether  the  magmatic  ore  deposits 
of  the  iron  oxids  and  the  metallic  sulfids  are  consolidated  during  the 
initial  or  late  magmatic  stages  has  not  yet  been  attacked  by  experimental 
work.  The  experimental  researches  of  Vogt 18  on  molten  sulfids,  valu- 


18  Day,  A.  L. — Some  mineral  relations  from  the  laboratory  viewpoint.     Bull. 
Geol.  Soc.  Amer.,  21,  141-178  (1910).  ie  Loc.  cit.  17  Loc.  cit. 

18  Vogt,  J.  H.  L. — Die  Silikatschmelzlosungen,  (1903-04). 


MAGMATIC  DIFFERENTIATION  AND  ORE  SEGREGATION  13 

able  as  they  are,  have  little  or  no  application  to  the  problem,  as  he  ex- 
perimented with  dry  melts,  and  no  attempt  was  made  to  experiment 
with  an  enclosed  system  into  which  mineralizers  were  introduced. 

Field  Investigations. — The  second  line  of  attack  is  the  study  and 
analysis  of  field  relations.  Are  the  magmatic  ores  always  located  at 
the  base  of  a  differentiated  igneous  rock?  If  this  occurs  in  some  cases, 
is  the  location  at  the  base  of  the  intrusive  rock  due  to  other  processes 
than  the  sinking  of  the  first-formed  crystals?  On  the  other  hand,  do 
the  ores  show  any  suggestive  relation  to  the  salic  differentiate  (peg- 
matite dikes,  etc.)  of  the  dominantly  femic  magma?  Are  the  magmatic 
ores  related  to  fractures  developed  after  partial  consolidation,  and  may 
these  fractures  concentrate  and  release  the  "acid  extracts,"  and  thus 
localize  the  ores?  Do  the  ores  migrate  into,  and  replace,  the  country 
rock?  Are  the  magmatic  ores  followed  in  some  cases  by  ore  deposits 
of  hydrothermal  origin,  into  which  they  may  grade? 

Apparently  field  evidence  alone  is  not  conclusive,  or  at  least  has 
been  variously  interpreted,  for  and  against,  the  magmatic  origin  of 
the  ores.  At  Sudbury,  the  location  of  the  ores  at  the  base  of  the  norite 
sill  has  been  considered  strong  evidence  of  magmatic  origin  during  the 
early  stages  of  consolidation.  On  the  other  hand,  Knight 19  has  re- 
cently presented  field  evidence  which  he  believes  is  proof  of  the  hydro- 
thermal  origin  of  the  ores.  The  field  evidence  regarding  the  deposits 
investigated  appears  to  us  to  be  of  a  corroborative  rather  than  of  a 
conclusive  nature. 

MICROSCOPIC  INVESTIGATION 

The  determination  of  the  age  of  the  ore-minerals  in  magmatic 
deposits  is  merely  one  phase  of  the  general  problem  of  the  determina- 
tion of  the  order  of  crystallization  of  minerals  in  igneous  rocks.  The 
problem  is  complex  and  difficult  as  far  as  the  early  silicates  are  con- 
cerned, as  is  witnessed  by  the  differences  of  opinion  expressed  in  recent 
contributions,20  and  by  the  doubt21  thrown  on  Rosenbusch's  fundamen- 
tal law  of  decreasing  basicity. 

These  difficulties  disappear,  however,  in  regard  to  the  late  mag- 
matic minerals,  for  by  microscopic  examination  of  both  polished  and 


19  Knight,  C.  W.— Origin  of  Sudbury  nickel-copper  deposits.  Eng.  and  Min. 
Jour.,  101,  811-812  (1916). 

20Bowen,  N.  L.— The  order  of  crystallization  in  igneous  rocks.  Jour.  Geol., 
20,  455-468  (1912). 

Ziegler,  V. — The  order  of  crystallization  in  igneous  rocks.  Jour.  Geol.,  21, 
181-185  (1913).  21  See  Vogt,  Die  Silikatschmelzlosungen,  1,  160. 


14  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

thin  sections  (see  page  74  for  methods  employed)  we  have  found 
evidence  that  the  ore-minerals  surround,  cut,  and  replace  the  earlier 
silicates. 

The  more  important  microscopic  investigations  of  certain  of  these 
types  of  ores  include  those  of  Beck,  Vogt,  Dickson,  Campbell  and 
Knight,  Howe,  and  Berg,  to  which  reference  is  made  in  the  appropriate 
places.  Many  of  the  conclusions  of  these  investigators  are  verified 
by  us. 

CONCLUSIONS  FROM  OUR  MICROSCOPIC  INVESTIGATIONS 

Some  of  the  important  facts  that  appear  to  us  to  be  established 
by  microscopic  study  for  both  the  sulfid  and  iron  oxid  ores  of  magmatic 
origin  are  as  follows: 

The  ore-minerals  are  the  final  magmatic  product,  and  are  formed 
later  than  the  magmatic  hornblende,  which  we  believe  to  be  produced  by 
magmatic  alteration. 

The  ores  replace  the  silicates  and,  in  general,  the  later- formed  ore- 
minerals  replace  the  earlier  ore-minerals. 

There  is  a  regular  order  of  formation  of  the  magmatic  minerals, 
which  shows  no  variation  in  the  deposits  studied.  For  the  nickel-copper 
group  of  sulfid  ores  it  is  as  follows:  (i)  silicates,  (2)  magnetite  and 
ilmenite,  (3)  pyrrhotite,  (4)  pentlandite,  and  (5)  chalcopyrite.  For  the 
chalcopyrite-bornite  group  the  order  is:  (i)  silicates,  (2)  magnetite  and 
ilmenite,  (3)  hematite,  (4)  pyrrhotite  (when  present),  (5)  chalcopyrite 
and  bornite.  All  alteration  minerals,  except  hornblende,  are  later  than 
the  above  mentioned  magmatic  ores.  In  some  cases  minor  amounts  of 
"rearranged  ores"  have  been  recognized,  but  the  extent  of  the  rearrange- 
ment is  surprisingly  small. 

From  the  field  relations  we  find  that  the  ores  may  be  followed  by 
pegmatite  dikes  (often  containing  ore-minerals)  and  by  later  series  of 
hydrothermal  ore-bearing  veins. 

THE  EVIDENCE  ON  WHICH  THE  ABOVE  CONCLUSIONS  ARE  BASED 
We  conclude  that  the  ores  are  later  than  the  silicates,  for  the  rea- 
son that  all  the  silicates  indiscriminately  occur  as  relicts  in  a  ground 
mass  of  ore.  The  ore-minerals  surround  the  silicates,  enter  along  the 
contacts  between  them,  cut  them,  and  penetrate  easily  cleavable 
minerals  such  as  biotite.  In  some  cases  they  cut  the  silicates  in  well 
defined  veinlets.  These  relations  are  explained,  in  part,  by  those  fav- 
oring an  early  magmatic  origin  of  the  ores  as  follows :  The  sulfid 
ores  remain  in  a  molten  condition  during  the  formation  of  the  primary 


MAGMATIC  DIFFERENTIATION  AND  ORE  SEGREGATION  15 

silicates    (we  add:    during  the   formation  of  the  late  magmatic  horn- 
blende), and  then  solidify. 

The  presence  of  hornblende,  however,  suggests  a  moderate  tem- 
perature (far  below  the  melting  point  of  the  sulfids)  and  the  presence 
of  water  22  and  other  mineralizers.  Further,  it  is  certain  that  the  ore- 
minerals  have  either  replaced  or  corroded  the  silicates.  This  is  proved 
by  "intersecting  structures"  23  and  by  the  fact  that  portions  of  crystals 
have  been  removed  and  their  place  taken  by  ore-minerals.  Berg,24  de- 
scribing the  magmatic  nickel-bearing  sulfid  ores,  states :  "Sie  (the  nickel- 
bearing  sulfids)  umschliessen  nicht  nur  gelegentlich  alle  anderen 
Gemengteile,  sie  resorbieren  dieselbe  nicht  nur  zu  rundlichen  Massen, 
sondern  sie  korrodieren  sie  auch  ofters,  indem  sie  ganz  nach  den  Gesetz- 
en  der  metasomatischen  Verdrangung  langs  Spaltrissen  und  mechan- 
ischen  Spalten  in  diese  eindringen." 

The  preservation  of  the  form  of  antecedent  crystals,  especially 
magnetite  and  olivine,  is  accomplished  by  selective  replacement.  Graphic 
texture  is  preserved  by  the  replacement  of  feldspar  of  the  quartz-feld- 
spar intergrowth.  Often  the  various  stages  in  the  replacement  of  a 
mineral,  from  incipient  to  complete,  may  be  noted. 

REPLACEMENT  PHENOMENA  IN  THE  MAGMATIC  ORES 
The  process,  however,  is  not  one  of  corrosion,  but  of  replacement. 
If  the  ores  were  molten,  corrosion  should  produce  metallic  silicates  by 
reaction.  No  such  metal-bearing  slag  is  found.  The  phenomena  are 
those  of  ordinary  replacement,  and  the  agency  that  brought  in  the  sulfids 
removed  the  dissolved  silicates,  all  of  which  indicates  active  mineral- 
izers.25 

The  regular  order  in  which  the  sulfid  minerals  are  deposited  one 
after  the  other,  and  the  fact  that  one  replaces  the  other,  indicates 
deposition  by  mineralizing  solutions,  and  not  the  intrusion  of  molten 
sulfids. 

22Harker,  A.— The  natural  history  of  igneous  rocks,  289  (1909). 
Bowen,  N.  L. — Loc.  cit,  41. 

23  Irving,  J.  D. — Replacement  ore  bodies  and  the  criteria  for  their  recognition. 
Econ.  Geol.,  6,  647  (1911). 

24  Berg.  G. — Mikroskopische  Untersuchung  der  Erzlagerstatten,  107-108  (1915). 

25  We  regard  sulfur  as  a  mineralizer  of  importance  in  the  magmatic  stages 
in  this  type  of  deposits.     Sulfur  is  not  usually  considered  a  mineralizer  by  petro- 
graphers,  but  it  is  recognized  to  be  such  by  de  Launay  and  the  French  school  gen- 
erally, beginning  with  de  Beaumont.     [Bull.  Soc.  Geol.  France,  4,  pt.  2,  1268  (1847).] 
The  recognition  of  sulfur  as  a  mineralizer  calls  attention  to  the  arbitrary  distinc- 
tion between  the  terms  "mineralizer"  and  "mineralization." 


16  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

This  last  point  may  be  met,  in  part,  by  Howe's  hypothesis  that 
the  molten  ores  are  mixtures  of  mutually  immiscible  sulfids,  and  that 
those  last  to  crystallize  penetrate  the  earlier  sulfids.  This  suggestion 
does  not  overcome  the  difficulties  raised  by  the  replacement  of  the  sili- 
cates by  ores,  and  antecedent  ore-minerals  by  later  ores,  without  the  for- 
mation of  reaction  rims.  The  absence  of  the  latter  shows  that  the 
replaced  material  is  removed  by  the  same  vehicle  that  brought  in  the 
ore. 

The  accumulating  evidence  of  the  low  temperatures 26  at  which 
the  final  stages  of  the  consolidation  of  an  intrusive  magma  take  place, 
discredits  the  notion  that  a  sulfid  melt,  similar  in  character  to  that  with 
which  we  are  familiar  in  the  reverberatory  furnace,  can  exist  after  the 
final  consolidation  of  the  silicates  of  the  magma.  Its  "molten"  condi- 
tion must  be  due  to  mineralizers,  in  such  amounts  that  the  character- 
istics of  the  mixture  are  those  of  a  high-temperature  solution  and  not 
of  a  melt.27 

We  can  find  no  support  whatever  for  the  idea  that  the  sulfids  sep- 
arated as  molten  mixtures  and  solidified  later. 

In  our  microscopic  studies  of  contact-metamorphic  deposits  we 
have  found  definite  evidence  that  the  ore-minerals  are  later  than  the 
high-temperature  silicates  such  as  garnet,  pyroxene,  etc.  Similar  data 
lead  us  to  believe  the  same  is  true  for  all  the  high-temperature  deposits. 

We  have  come  to  the  conclusion,  therefore,  that  the  formation  of 
sulfids  takes  place  at  a  late  stage  in  all  types  of  high- temperature  de- 
posits, probably  not  higher  than  3OO°-4OO°  C.  In  this  estimate  we  differ 
greatly  from  Lindgren,28  especially  for  the  magmatic  deposits,  who  be- 
lieves that  the  ores  are  about  contemporaneous  with  the  high-tempera- 
ture silicates. 


2«  Marker,  A.— Loc.  cit.,  184-188. 

27  Beck  calls  attention  to  the  physical  improbability  of  molten  sulfids  entering 
into  the  cooler  country  rock  adjacent  to  the  intrusives,  and  states  that  the  "cor- 
rosion" of  the  silicates  has  been  caused  by  water  solutions,  and  believes  that  in 
many  cases  the  ores  have  formed  after  the  regional  metamorphism  of  the  gabbros, 
and  are  younger  than  the  hornblende  and  garnet.     [Lehre  von  den  Erzlagerstatten, 
dritte  Auflage,  erster  Band,  72-73   (1909).] 

28  Lindgren   (Mineral  Deposits,  188)  gives  the  ranges  of  temperature  for  the 
high-temperature  deposits  as  follows : 

Magmatic  deposits,  700°  to  1500°  C. 

Contact  deposits  and  allied  veins ;  pegmatites,  3OO°±  to  8oo°±. 

Vein  and  replacement  deposits  formed  at  great  depths,  3OO°±  to  5oo°±. 


MAGMATIC  DIFFERENTIATION  AND  ORE  SEGREGATION  17 

ALTERATION  AND  LATER  REARRANGEMENT  IN  MAGMATIC  ORES 

The  sharp  sulfid  veinlets  will  be  recognized  by  all  as  later 
than  the  silicates  they  cut.  They  furnish  no  stronger  proof,  however, 
of  the  late  origin  of  the  ores  than  the  larger  scale  relations,  such  as  the 
surrounding  and  penetration  of  the  silicates  by  the  ore-minerals. 
The  well-defined  veinlets  in  the  <  magmatic  ores  have  been  ex- 
plained as  "later  rearrangements."  Lindgren29  states,  in  discussing 
Uglow's  description  of  the  replacement  and  vein  phenomena  shown  in 
the  nickel-bearing  sulfids  at  the  Alexo  mine,  Ontario :  "Here,  as  in  so 
many  other  cases,  secondary  changes  appear  to  have  been  confused 
with  primary  deposition."  Coleman,  writing  of  the  Sudbury  ores, 
states:30  "That  there  has  been  a  certain  amount  of  solution  and  rede- 
position  in  many  of  the  ore  deposits  is  admitted  by  all,  but  this  was 
of  the  nature  of  a  rearrangement  of  the  minerals  of  the  rock." 

We  devoted  considerable  time  to  the  investigation  of  the  veinlets, 
vein-like  replacements,  and  alteration  products  accompanying  the  ores, 
in  order  to  determine  the  extent  of  these  later  rearrangements.  Numer- 
ous sulfid  veinlets  occur  in  certain  of  the  Sudbury  ores,  but  the  exami- 
nation of  these  shows  no  indication  of  more  than  one  generation  of  ore- 
minerals.  These  veinlets  lead  out  from  large  sulfid  masses  and  show 
no  rearrangement,  nor  are  they  accompanied  by  contemporaneous 
alteration  products,  as  is  the  insignificant  second  generation  of  ore-min- 
erals occasionally  met  with.  Where  the  sulfid  veinlets  cut  a  zone  of 
reticulated  fractures  filled  with  alteration  minerals,  the  sulfids  do  not 
penetrate  these  fractures,  and  therefore  they  were  deposited  before  the 
alteration.  No  greater  amount  of  alteration  is  found  in  the  ores  with, 
or  in  the  neighborhood  of,  the  veinlets  than  elsewhere.  The  sulfid  vein- 
lets  are  found  in  the  normal  norite,  in  the  "acid  material,"  and  in  the 
"basic"  segregations,  so  that  the  ore  is  younger  than  all  these  three  types 
of  rocks. 

None  of  the  magmatic  ores  are  entirely  free  from  the  products 
of  secondary  alteration,  but  all  the  secondary  minerals  except  the  late 
magmatic  hornblende  are  definitely  later  than  the  ore-minerals.  In  a 
single  slide  a  portion  of  the  ore  may  be  unaccompanied  by  alteration 
products,  and  another  portion  may  be  surrounded  and  cut  by  later 
secondary  silicates;  but  the  relations  of  ore  to  gangue  are  the  same 
in  the  altered  portions  as  in  the  fresh,  and  there  is  often  no  indication 

29  Lindgren,  W. — Mineral  Deposits,  765. 

80  Coleman,  A.  P. — The  nickel  industry,  Canada  Dept.  Mines,  Report  170, 
P-  3i  (1913)- 


18  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

of  secondary  migration  of  ore-minerals,  even  where  they  are  cut  by 
veinlets  of  chlorite.  The  striking  thing  about  the  magmatic  ores  in 
general  is  the  slight  amount  of  rearrangement  they  have  undergone. 

The  results  of  our  study  of  the  ores  of  the  Alexo  mine,  Canada, 
are  of  interest  in  showing  that,  altho  the  silicates  are  intensely  altered, 
and  completely  serpentinized,  nevertheless  the  ore  is  only  slightly 
affected.  Here  the  serpentinization  is  subsequent  to  ore  formation,  as 
shown  by  veinlets  of  serpentine  cutting  the  main  ore  masses.  Two  gener- 
ations of  serpentinization  are  recognized ;  the  first  accomplished  the  seg- 
regation of  insignificant  veinlets  and  specks  of  chalcopyrite  within  the 
area  serpentinized,  but  apparently  did  not  modify  even  the  outline  of  main 
masses  of  ore;  the  second  generation  of  serpentine  is  accompanied  by 
still  more  minute  scattered  microscopic  specks  of  ore.  Only  a  small 
portion  of  the  total  ore  suffered  rearrangement  during  the  process  of 
serpentinization. 

ABSENCE  OF  PNEUMATOLYTIC  AND  HYDROTHERMAL  ALTERATION  DURING 

ORE  DEPOSITION 

One  of  the  characteristics  of  magmatic  ores  that  has  long 
been  emphasized  in  the  literature  is  the  lack  of  secondary  silicates 
produced  by  pneumatolytic  and  hydrothermal  processes.  In  a  few  cases, 
however,  the  ores  are  accompanied  by  high-temperature  minerals,  such 
as  garnet  and  tourmaline,  and  show  gradations,  therefore,  towards  the 
groups  of  high-temperature  epigenetic  ore  deposits.  Our  work  shows 
that  hydrothermal  alteration  is  invariably  later  than  the  period  of  mag- 
matic ore  formation,  and  therefore  emphasizes  the  lack  of  alteration 
during  the  formation  of  the  ores.  It  has  been  argued  that  if  mineral- 
izers  or  solutions  are  involved  in  the  formation  of  the  magmatic  ores, 
they  must  of  necessity  not  only  dissolve  the  rock  and  deposit  the  ore, 
but  must  also  react  with  the  rock-forming  minerals  to  produce  second- 
ary silicates.  For  example,  Vogt 81  was  at  first  inclined,  from  field  and 
microscopic  data,  to  attribute  the  origin  of  the  nickeliferous  sulfids  of 
Norway  to  "pneumatolytic"  action ;  but  the  lack  of  the  products  of 
destructive  pneumatolysis  evidently  caused  him  to  abandon  the  idea 
that  mineralizers  are  the  agents  of  ore  concentration,  and  he  adopted 
the  concept  of  an  intrusive  sulfid  magma. 

The  idea  that  alteration  invariably  accompanies  ore  deposition  is 
probably  due  to  a  considerable  extent  to  the  emphasis  given  rock  alter- 


81  [Geol.   Foren.   Forh,   1883]    cited   by   Beyschlag,   Krusch,   Vogt,   Erzlager- 
statten,  p.  285. 


MAGMATIC  DIFFERENTIATION  AND  ORE  SEGREGATION  19 

ation  in  the  classic  paper  of  Lindgren.32  As  a  matter  of  fact,  a  careful 
microscopic  examination  by  us  of  many  types  of  non-magmatic  ores 
has  discovered  veinlets  or  portions  of  veinlets  along  which  the  rock  is 
little  altered;  and  where  the  rock  is  affected,  this  alteration  may 
Ue  earlier  or  later  than  the  deposition  of  the  sulfids,  and  not 
connected  directly  with  it.  In  replacement  deposits  in  lime- 
stone, the  most  easily  altered  of  all  rocks,  the  ore  often  lies 
in  contact  with  the  limestone  without  intervening  secondary  minerals. 
A  priori,  no  reason  can  be  found  why  the  composition  and  temperature 
of  the  ore-forming  solutions  in  general  should  not  be  such  that  solution 
of  the  rock-minerals  and  deposition  of  the  ore-minerals  may  take  place 
without  the  formation  of  secondary  silicates.  A  fortiori,  this  would  be 
expected  in  the  case  of  the  ores  under  discussion.  In  the  late  stages 
of  the  consolidation  of  igneous  rocks,  constructive  action  of  mineral- 
izers  aids  and  controls  the  formation  of  minerals  without  the  develop- 
ment of  secondary  silicates.  We  have  much  to  learn  from  the  French 
scientists  of  the  importance  of  mineralizers33  in  the  crystallization  of 
igneous  rocks. 

Extensive  destructive  pneumatolysis  often  occurs  in  connection 
with  certain  stages  in  the  formation  of  pegmatite  dikes  and  high-tem- 
perature veins,  and  results  in  the  development  of  such  minerals  as 
quartz,  muscovite,  tourmaline,  topaz,  scapolite,  etc.  The  temperature 
and  character  of  the  solutions  and  mineralizers  are  such  that  they  are 
not  in  equilibrium  with  the  minerals  attacked,  and  they  contain  chem- 
ically active  gases,  such  as  boron,  chlorin,  fluorin,  etc.  They  therefore 
develop  in  the  country  rock  the  complicated  set  of  silicates  mentioned. 


32  Lindgren,   W. — Metasomatic  processes   in  fissure-veins.     Trans.   Am.   Inst. 
Min.  Eng.,  30,  578-692  (1900). 

33  There  is  a  striking  difference  between  the  German  and  American  schools 
of  petrography  and  the  views  of  the  French  scientists.     The  former  have  inter- 
preted the  conditions  governing  the  crystallization  of  deep-seated  magmas  in  the 
light  of  observations  on  surface  lavas  and  experiments  on  anhydrous  melts.     The 
latter  have  been  impressed  with  the  fumarolic  action  around  volcanic  vents,  and 
have  therefore  concluded  that  mineralizers  are  important  components  in  intrusive 
magmas.     [Lacroix,  A. — Les  mineraux  des  fumerolles  de  1'eruption  du  Vesuve  en 
avril  1906.    Bull.  Fr.  Soc.  Min.,  30,  219-266  (1907).]     Extreme  views  in  regard  to 
the  role  of  mineralizers  have  been  expressed  by  de  Launay  [Traite  de  Metallogenie. 
Cites  mineraux  et  metalliferes,  1,  chap.   I    (1913)]  who  has  developed  a  theoret- 
ical concept  of  the  constitution  of  the  earth  involving  a  central  metallic  core  with 
a  superficial  slag  or  silicate  crust.     He  believes  that  mineralizers  react  upon  the 
"slag,"  producing  granite  and  other  rocks,  and  form  various  types  of  ore  bodies 
from  the  metals  they  have  "extracted"  from  the  metallic  core. 


20  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

We  conclude  that  the  magmatic  ores,  in  contrast  with  the  peg- 
matites and  "high-temperature  veins,"  occur  in  rocks  which  are  little 
affected  by  the  destructive  action  accompanying  the  latter. 

The  lack  of  alteration  during  the  formation  of  the  magmatic  ores 
in  "basic"  rocks  is  probably  the  result  of  chemical  composition  rather 
than  of  temperature.  The  mineralizers  of  the  "basic"  rocks  containing 
the  magmatic  sulfids  produce  hornblendization  and  biotitization,  while 
those  of  "acid"  rocks  produce  marked  destructive  effects  (greisenization, 
tourmalinization,  silicification,  contact  action,  etc.). 

The  lack  of  alteration  during  ore  formation,  the  fact  that  ore  forma- 
tion is  often  followed  by  the  intrusion  of  pegmatitic  differentiates,  also 
the  fact  that  the  ores  are  limited  to  the  parent  "basic"  rock  and  migrate 
into  the  intruded  rock  only  to  a  very  minor  degree,  are  the  chief  character- 
istics of  this  definite  and  recognizable  type  of  ore  which  has  been  desig- 
nated as  magmatic.  Therefore  we  may  well  retain  this  term,  even  tho 
certain  misconceptions  have  been  attached  to  it,  such  as  consolidation  in 
the  early  molten  stage,  injection  of  molten  sulfids,  etc. ;  recognizing  that 
the  microscopic  examination  definitely  proves  only  that  the  ores  are 
later  than  the  primary  silicates  and  earlier  than  the  hydrothermal  sili- 
cates, and  also  that  the  period  of  metallization  is  further  removed  from 
the  main  stages  of  rock  consolidation  than  is  generally  believed.33* 

MAGNETITE-!LMENITE  MAGMATIC  ORES 

We  have  studied,  chiefly,  the  magmatic  sulfid  ores,  and  for  these 
there  is  no  doubt  that  the  ores  are  later  than  the  silicates.  The  same 
is  true  for  the  magnetite-ilmenite  ores  we  have  examined;  and  a  review 
of  the  literature  of  these  deposits  containing  accurate  microscopic  de- 
scriptions shows  this  to  be  true  for  all  the  iron  ores  of  this  type,  altho 
the  bearing  of  this  fact  on  the  theories  of  magmatic  differentiation  does 
not  appear  to  have  been  duly  emphasized  or  appreciated. 

EUHEDRAL  MAGNETITE  FORMED  AT  A  LATE  MAGMATIC  STAGE 
Berg84  recognizes  in  the  majority  of  magmatic  iron-oxid  deposits 
two  generations  of  magnetite:  euhedral  magnetite  earlier  than  the  sili- 

880  At  various  times  during  the  progress  of  our  studies,  we  have  considered 
the  advisability  of  discarding  the  term  "magmatic  ore"  in  favor  of  some  such  name 
as  epimagmatic.  The  fact  that  we  have  not  been  able  to  discover  ores  that  show 
evidence  of  having  been  formed  during  the  early  stages  in  the  consolidation  of  the 
magma,  that  the  activity  of  mineralizers  increases  as  crystallization  progresses,  and 
that  any  division  between  earlier  and  later  minerals  would  be  arbitrary,  has  de- 
terred us  from  suggesting  any  departure  from  the  present  nomenclature,  altho  such 


MAGMATIC  DIFFERENTIATION  AND  ORE   SEGREGATION  21 

cates,  and  larger  areas  of  magnetite  later  than,  and  "corroding,"  the 
silicates.  As  magnetite  is  also  a  constituent  of  the  magmatic  sulfid  ores, 
and  as  it  occurs  both  as  euhedral  crystals  and  anhedral  masses  replacing 
the  silicates,  we  were  able  to  investigate  the  question  as  to  the  occur- 
rence of  the  two  generations  of  magnetite.  We  find  that  there  are  all 
gradations  between  small  euhedral  crystals  and  the  large  irregular 
areas  of  magnetite.  The  points  of  the  larger  masses  may  show  crystal 
faces  where  they  penetrate  well  into  the  silicate  minerals.  The  mag- 
netite appears  to  develop  irregular  forms  where  it  surrounds  the  sili- 
cates, apparently  following  the  lines  of  least  resistance  along  the  min- 
eral boundaries,  but  develops  euhedral  forms  within  the  silicates,  where 
no  line  of  weakness  disturbs  its  tendency  to  crystallize  regularly.  We 
believe  that  the  magnetite  in  the  magmatic  sulfid  ores,  and  probably  in 
the  magmatic  ores  in  general,  is  all  deposited  later  than  the  silicates. 

OTHER  ACCESSORY  MINERALS  OF  PROBABLE  LATE  MAGMATIC  ORIGIN 

The  accessory  minerals  of  igneous  rocks,  such  as  magnetite,  ilmen- 
ite,  apatite,  titanite,  zircon,  etc.,  are  considered  by  petrographers  to  be  the 
first  to  crystallize  in  igneous  rocks,  on  account  of  their  euhedral  form, 
and  their  occurrence  within  the  silicates  that  make  up  the  mass  of  the 
rock.  As  the  effect  of  mineralizers  is  not  generally  recognized  in  devel- 
oping new  minerals  in  the  igneous  rocks  the  possibility  has  not  been  con- 
sidered that  these  minerals  form  in  the  later  magmatic  stages  by  replace- 
ment.35 Euhedral  crystals  of  pyrite  in  igneous  and  sedimentary  rocks 
and  of  garnet  and  magnetite  in  metamorphic  rocks,  are  found  unaccom- 
panied by  any  adjacent  alteration  zone.  In  a  similar  manner  we  believe 
that  euhedral  magnetite,  and  probably  other  accessory  minerals,  are  the 
product  of  mineralizers  developed  during  the  late  magmatic  stage. 

The  high-temperature  minerals  forming  the  bulk  of  the  igneous 
rocks  are  formed  at  an  early  stage.  The  accessory  minerals  present  in 
small  amounts  are  not  formed  until  a  late  stage  and  then  under  the 
influence  of  mineralizers.  The  difficulty  is  thus  avoided  as  to  how  the 
magma,  at  an  early  stage,  could  become  saturated  in  relatively  insoluble 
compounds  present  in  small  amounts,  and  how  these  again  could  make 
their  appearance  in  large  amounts  among  the  last  products  of  consoli- 
dation. 


a  change  might  assist  in  clearing  up  misconceptions  in  regard  to  the  magmatic  ores 
and  the  processes  of  magmatic  differentiation.  34  Loc.  cit.,  102,  105. 

35  "If  crystals  of  one  primary  mineral  completely  enclose  crystals  of  another, 
the  evidence  of  their  relative  age  is  absolute."     (Harker,  loc.  cit.,  179.) 


22  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

STAGES  RECOGNIZED  IN  THE  FORMATION  OF  MAGMATIC  ORES 

From  the  above  discussion  it  is  clear  that  we  conceive  of  the  pro- 
cess of  formation  of  plutonic  rocks  as  consisting  of  stages,  and  that 
rock  differentiation  and  ore  formation  are  the  results  of  an  orderly 
series  of  events. 

The  process  varies  in  detail  with  the  composition  and  size  of  the 
individual  intrusive  masses  undergoing  crystallization.  The  stages  in 
the  norites  and  gabbros  which  contain  the  magmatic  sulfid  ores  are 
as  follows: 

(i).  The  first  minerals  to  form  are  olivine,  the  pyroxenes,  and  the 
feldspars. 

(2).  Magmatic  alteration  of  the  silicates  often  takes  place  prior 
to  the  formation  of  the  ore-minerals.  The  most  common  change  is 
that  of  pyroxene  to  hornblende  (not  uralite).  The  not  uncommon  horn- 
blende gabbro,  for  example,  may  be  developed  by  this  late  magmatic 
process,  for  the  hornblende  has  probably  been  formed  at  the  expense 
of  pyroxene. 

(3).  Later  magmatic  products  include  interstitial  pegmatitic  mater- 
ial, interstitial  quartz,  and  occasionally  tourmaline,  garnet,  analcite, 
epidote,  and  calcite. 

(4).  The  introduction  of  the  ores  by  mineralizers  is  later,  in  gen- 
eral, than  the  minerals  of  group  (3)  and  is  unaccompanied  by  any  sec- 
ondary silicates. 

(5).  The  pegmatite  dikes,  found  in  the  neighborhood  of  almost  all 
of  the  magmatic  sulfid  ores,  are  often  later  than  the  magmatic  deposits  of 
the  basic  rock  itself. 

(6).  Hydrothermal  alteration  subsequent  to  magmatic  ore  deposi- 
tion includes  the  development  of  chlorite,  tremolite,  anthophyllite,  seri- 
cite,  and  serpentine.  In  general,  hydrothermal  alteration,  altho 
seldom  lacking,  is  insignificant  compared  with  that  developed  in  con- 
nection with  deposits  of  other  types  in  igneous  rocks,  such  as  those 
of  Butte,  Bingham,  etc.  It  often  does  not  accomplish  any  rearrange- 
ment of  the  ore,  altho,  in  some  cases,  insignificant  amounts  of  pent- 
landite,  chalcopyrite,  chalcocite  and  covellite  are  formed. 

(7).  At  a  later  stage,  downward  enrichment  and  oxidation  may 
take  place.  Magmatic  deposits,  in  all  cases  examined,  owe  their  metallic 
content  to  the  original  magmatic  minerals  and  not  to  later  introduced 
sulfids. 


PART  II. 

DESCRIPTION  OF  THE  VARIOUS  DEPOSITS  OF 
THE  MAGMATIC  ORES 


GROUP  I.    THE  NICKEL-  AND  COPPER-BEARING 
PYRRHOTITIC  DEPOSITS 


SUDBURY,  CANADA 
GEOLOGY 

Those  who  favor  the  hypothesis  that  the  Sudbury  ores  separated  out 
of  the  magma  as  an  early,  or  the  earliest,  constituent,  and  sank  and  col- 
lected by  gravitative  differentiation,36  cite  field  relations  as  proving  their 
contention.  In  spite  of  the  voluminous  literature  in  which  these  rela- 
tions have  been  described,  there  are  still  many  points  in  regard  to  which 
no  agreement  has  been  reached.  A  discussion  of  the  genesis  of  these 
deposits  is  therefore  incomplete  without  a  summary  of  the  established 
field  relations  and  a  statement  of  the  problems  that  are  unsolved  as  yet. 
Fortunately  Coleman  37  has  mapped  in  detail  both  the  upper  and  lower 
margins  of  the  laccolithic  sheet,  and  given  much  information  in  regard 
to  the  structure  of  the  individual  deposits.  The  accompanying  map  (fig. 
i)  is  drawn  from  a  photographic  reduction  of  his  large-scale  map. 

As  is  well  known,  the  pyrrhotite-pentlandite-chalcopyrite  ore 
bodies  of  the  Sudbury  district  occur  at  the  base  of  a  great  sheet  of 
"norite-micropegmatite."  As  shown  on  the  map,  the  sheet  is  of  unusual 
regularity  for  an  intrusive  mass  of  its  size  and  character.  It  is  overlain 
by  9000  feet  of  Upper  Huronian  sandstones,  shales,  conglomerates,  and 
tuffs,  which  make  a  gentle  syncline  concordant  in  dip  with  the  norite 
sheet,  and  underlain  by  a  sedimentary  series  of  great  thickness  (30,000 
feet  according  to  Coleman),  predominantly  of  quartzite  with  included 
older  basic  irruptives.  These  older  strata,  "the  Sudbury  series"  of  early 
Huronian  or  pre-Huronian  age,  dip  regularly  to  the  southeast,  averaging 

36  Coleman,  A.  P. — The  nickel  industry.  Can.  Dept.  of  Mines,  Mines  Branch. 
Publ.  170  (1913)- 

Barlow,  A.  E— Geol.  Surv.  Can.,  14,  pt.  H,  1-124  (1904). 
37Loc.  cit. 


w 

'|i  ir 

ilflUfSilt 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  25 

45°.  The  "nickel  eruptive,"  as  the  norite  sheet  has  been  named  by 
Coleman,38  therefore,  has  taken  advantage  of  the  unconformity  between 
the  older  and  younger  sedimentary  series,  and  has  lifted  the  latter  on  its 
back  without  disturbing  it  otherwise  than  to  develop  a  gentle  spoon- 
shaped  syncline. 

No  accurate  sections  across  the  syncline  have  been  made,  but  judging 
from  the  areal  map  and  the  descriptions  and  sketches  of  the  ore  bodies 
located  at  the  contact  between  the  nickel  eruptive  and  the  underlying 
formations,  the  dip  of  the  eruptive  sheet  at  its  basal  contact  is  40°  to 
45°  at  the  surface,  generally  decreasing  to  30°  to  35°  at  a  depth  of  about 
1000  feet. 

Coleman  39  has  pictured  in  the  following  vivid  language  his  concept 
of  the  manner  in  which  the  great  laccolith  forced  its  way  into  the  sedi- 
mentary rocks : 

"After  the  succession  of  sediments  just  mentioned  had  been  de- 
posited, the  vast  mass  of  molten  rock  of  the  nickel  eruptive  ascended, 
mostly  from  beneath  an  area  near  the  middle  of  the  southern  range,  as 
will  be  shown  later.  As  the  molten  magma  welled  up  from  below,  the 
crystalline  rocks  forming  the  roof  of  the  great  crucible  gradually  col- 
lapsed as  a  block  twelve  to  fifteen  miles  long  and  several  miles  broad, 
giving  rise  to  extensive  faulting  and  fissuring.  .  .  . 

"The  eruptive  sheet  cooled  extremely  slowly,  partly  because  of  the 
great  bulk  of  molten  material  and  partly  because  of  the  thick  mantle  of 
sedimentary  rocks  above ;  and  during  the  cooling  much  of  the  ore  sank 
to  the  bottom,  though  its  upper  part  remained  mixed  with  norite,  which 
finally  blended  into  micropegmatite  or  granite  on  top,  the  three  materials 
arranging  themselves  according  to%  their  specific  gravities.  .  .  . 

"The  norite-micropegmatite  sheet  is  one  of  the  largest  laccolithic 
sheets  known,  containing  not  less  than  600  cubic  miles  at  the  present 
time,  and  probably  having  had  a  much  greater  bulk  in  the  beginning." 

Subsequent  to  the  intrusion  of  the  nickel  eruptive,  both  granite 
masses  and  dikes  and  basic  dikes  were  intruded  along  the  under  contact 
of  the  sill,  and  both  "acid"  and  "basic"  dikes  cut  the  ore  bodies. 

One  of  the  most  important  problems,  as  yet  unsolved,  is  the  origin 
of  the  "acid  material,"  largely  quartz  and  feldspar,  frequently  developing 
marked  micropegmatitic  structure,  that  occurs  enclosed  in  many  of  the 

38  Coleman,  A.  P.— The  Sudbury  laccolithic  sheet.  Jour.  Geol.,  15,  752-782 
(1907). 

39  Canadian  Dept.  Mines,  Pub.  170,  p.  10  (1913)- 


26  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

ore  bodies.  Equally  important  is  the  determination  of  the  relation  of 
the  "later  granite"  to  the  norite.  Coleman  has  arranged  the  granitic 
rocks  in  three  groups  (see  geological  map,  page  24).  (i)  Granite  and 
granite-gneiss  younger  than  (intrusive  into)  the  Sudbury  series  and 
older  than  the  norite;  (2)  granite  mostly  older  than  norite  which  was 
mapped  without  discrimination  between  "the  older"  and  "the  younger" 
groups  which  are  of  widely  different  origin;  (3)  granite  younger  than 
norite,  a  small  area  of  which  is  shown  near  the  Murray  mine.  The  lack 
of  detailed  field  and  microscopic  study  prevents,  for  the  present,  the 
final  answer  to  the  questions  raised  above.  Fortunately,  however, 
Knight40  has  been  studying  these  questions  in  the  course  of  his  detailed 
field  work,  and  we  look  forward  to  the  publication  of  his  conclusions 
with  great  interest. 

In  regard  to  the  micrographic  segregations  and  inclusions  of  gran- 
itic material  in  the  ore,  we  believe  with  Coleman41  that  they  are  differ- 
entiates of  the  norite. 

The  micrographic  structure  is  typical  of  the  upper  portion  of  the 
"nickel  intrusive."  Its  occurrence  in  the  ores  as  blebs  and  interstitial 
material,  as  well  as  large  masses,  and  the  variations  shown  in  a  single 
thin  section,  suggest  that  both  the  norite  and  the  "acid"  material  are 
differentiates  of  a  common  magma. 

In  this  regard  Howe42  states: 

"The  attractive  possibility  has  been  considered  that  the  silicious 
material  associated  with  the  sulphides  might  represent  a  residual  portion 
of  the  magma  from  which  the  sulphides  are  supposed  to  have  been 
derived.  .  .  .  The  microscope,  however,  shows  over  and  over 
again  the  absolute  similarity  of  the  acid  inclusions  to  the  foot-wall 
granite." 

However,  the  foot-wall  granite  to  which  he  refers  may  well  be 
the  "later  granite."  Specimens  sent  us  by  Dr.  Knight  from  the 
Creighton  mine,  the  locality  which  Howe  studied,  are  decidedly  micro- 
graphic.  Until  further  data  are  available,  the  reasonable  hypothesis  is 
that  the  "acid"  blebs,  segregates  and  inclusions  in  the  Sudbury  ore 
bodies,  as  well  as  the  marginal  masses  of  the  later  granite,  are  differen- 
tiates of  the  parent  norite  magma.  In  many  other  localities,  granitic 
differentiates  are  found  inclosed  in,  or  at  the  margins  of,  the  norite 
intrusives  (see  the  map  of  the  Ringerike  district,  Norway,  page  49). 
At  Sudbury,  where  all  the  phenomena  have  taken  place  on  a  large  scale, 

40  Loc.  cit.  This  article  contains  a  preliminary  statement  of  certain  of  the 
field  relations.  41  Econ.  Geol.,  10,  391-392  (1915).  42  Loc.  cit.,  520-521. 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  27 

we  should  expect  the  "acid"  differentiates  to  appear  in  large  amounts. 
The  fact  brought  out  by  Knight  that  dikes  of  the  later  granite  cut  the 
norite,  and  also  that  the  ore  rests  upon  a  footwall  of  younger  granite, 
does  not  prove  that  the  ores  are  not  magmatic.  The  norite  and  granite 
differentiates  may  be  nearly  contemporaneous,  and  the  "late  magmatic" 
ores  are,  in  some  localities,  earlier  than,  and  in  other  places  later  than,  the 
"acid"  differentiates.  Evidence  of  extensive  differentiation  is  found  gen- 
erally in  rocks  associated  with  magmatic  ores. 

Knight,  42°  has  also  emphasized  that  the  Sudbury  ores  have  wan- 
dered out  into,  and  replaced,  the  schists  found  at  the  contact  with  the 
norite. 

We  are  indebted  to  Dr.  C.  W.  Knight  for  specimens  of  several 
types  of  sulfid  bearing  "greenstone  schists."  These  laminated  rocks 
consist  largely  of  biotite,  hornblende,  actinolite,  chlorite,  clinozoisite, 
quartz,  and  plagioclase.  One  of  these,  a  chlorite-actinolite  gneiss  from 
the  Garson  mine,  is  shown  in  fig.  10.  The  ore-minerals,  pyrrhotite  and 
chalcopyrite,  occur  in  linear  areas  parallel  to  the  plane  of  schistosity. 
Under  the  microscope  the  relations  of  the  ore-minerals  to  the  silcates  are 
practically  the  same  as  in  the  igneous  rocks  examined.  The  sulfids  sur- 
round the  silicates,  and  evidently  replace  them  to  some  extent,  and  minute 
sulfid  veinlets  definitely  cut  the  silicates.  Alteration  products  are  prac- 
tically absent. 

This  shows  that  the  magmatic  ores  migrate  to  some  extent  into  the 
country  rocks,  and  apparently  show  the  same  replacement  phenomena 
as  are  exhibited  by  the  ores  in  the  igneous  rocks. 

STRUCTURE  OF  THE  ORE  BODIES 

Coleman,  in  his  recent  description  of  the  Sudbury  region,  recognizes 
the  following  types  of  ore  bodies : 

I.  Marginal  Deposits. — These  are  sulfid  segregations  collected  at  the 
base  of  the  main  norite  sheet.  They  gradually  fade  out  above  into  barren 
country  rock,  and  often  show  brecciation,  fissuring,  and  "later  reconcen- 
tration  of  ores",  especially  along  the  foot-wall,  which  is  always  a  pro- 
nounced fissure.  Faulted  marginal  deposits  have  suffered  brecciation 
and  faulting  in  their  upper  portions,  and  the  ore  has  "wandered  into  the 
fissures  between  the  blocks,  either  at  the  time  as  molten  sulphides,  or 
later  through  water  transport.  As  chalcopyrite  is  everywhere  the  more 
transferable  of  the  sulphides,  it  has  entered  the  fissures  more  largely 
than  the  pyrrhotite.  Unusually  large  amounts  of  quartz,  carbonates,  and 


42«Loc.  cit. 


28  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

sulphides  of  zinc  or  lead  are  found  in  these  two  mines  (Crean  Hill  and 
Garson  mines)  as  a  result  of  circulating  waters,  and  these  later  pro- 
cesses have  played  a  larger  part  than  in  most  other  ore  bodies  of  the 
region,  whether  marginal  or  offset."43 

2.  Columnar  Offset  Deposits. — These  remarkable  ore  deposits,  the 
most  notable  of  which  occur  in  the  Copper  Cliff  mine,  are  great  cylin- 
drical ore  shoots. 

"In  the  last  report  on  the  nickel  region  the  Copper  Cliff  deposit 
was  known  to  go  down  for  1,000  feet  without  interruption,  as  a  rude 
oval  pipe  with  diameters  varying  from  50  to  200  feet,  and  a  dip  of 
77l/2°  to  the  northeast. 

"Since  that  time  the  two  ore  bodies  of  the  Victoria  mine,  though 
smaller  in  diameter,  have  been  followed  to  the  depth  of  1,400  feet  with 
no  indication  that  they  may  not  continue  indefinitely.  These  two  small 
cylinders  of  ore,  more  than  1,400  feet  in  length  and  close  together,  but 
never  meeting,  are  not  at  all  easy  to  account  for  on  any  other  theory 
than  the  magmatic  one,  and  this  continuance  to  so  great  a  depth  was 
not  anticipated  in  earlier  studies  of  the  region.  .  .  . 

"There  is  usually  more  evidence  of  water  action  than  in  the  mar- 
ginal mines,  and  often  a  certain  amount  of  quartz  and  of  rusty  weather- 
ing carbonates  is  mixed  with  the  ore,  probably  as  later  effects  of  mag- 
matic waters."  44 

The  following  ingenious  explanation  of  these  columnar  ore  shoots 
as  dikes  or  apophyses  running  out  from  the  main  laccolith  is  favored 
by  Coleman :  ".  .  .  it  is  possible  that  the  most  fluid  part  of  the  magma, 
the  pyrrhotite-norite,  entering  all  the  fissures  produced  by  the  collapse 
of  the  underlying  rock,  rose  from  beneath  under  hydraulic  pressure  and 
was  able,  in  a  sense,  to  drill  holes  up  through  the  crushed  zones  of  rock 
above."  « 

3.  Parallel  Offsets. — These  include  the  great  Frood-Stobie  ore  de- 
posit described  by  Coleman  as  follows: 

"The  Frood-Stobie  offset  runs  nearly  parallel  to  the  basic  edge, 
but  at  a  distance  of  from  24  of  a  mile  to  1^2  miles  to  the  southeast. 
The  ore  more  nearly  resembles  that  of  a  marginal  deposit  than  that  of 
the  ordinary  offsets ;  and  the  ore  body  dips  at  an  angle  of  60°  toward 
the  basic  edge.  It  is  a  long  irregular  sheet  enclosing  much  rock,  and 
its  connexion  with  the  edge  of  the  norite  is  probably  at  a  considerable 
depth  below  the  surface.  The  margin  of  the  norite  parallel  to  it  shows 
comparatively  little  ore,  the  sulphides  belonging  to  it  having  been  drained 

43  Loc.  cit,  35.  44  Coleman.— Loc.  cit.,  36-37.  45  Loc.  cit.,  37. 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  29 

off  through  a  complex  set  of  fissures  to  the  Frood-Stobie  deposit.  The 
ore  is  known  by  diamond  drilling  to  extend  northwest  beneath  the  coun- 
try rocks  to  a  depth  of  more  than  1,000  feet,  and  at  the  lower  points 
it  distinctly  flattens  toward  the  basic  edge  of  the  norite.  No  other 
deposit  of  this  type  has  so  far  been  discovered;  but  the  Frood-Stobie 
belt  of  ore  is  so  important  and  so  very  distinct  from  the  other  types 
that  it  deserves  a  place  by  itself." 

From  the  above  descriptions  it  appears  that  the  parallel  offsets  are 
mineralized  dikes  or  sills  parallel  to  the  main  foot-wall  contact  and  dip- 
ping toward  it. 

From  the  quotations  given  above,  it  will  be  seen  that  there  are 
evidences  of  hydrothermal  deposition  of  ore  and  gangue  minerals  dis- 
tinctly later  than  the  main  ore  mass  of  the  various  types  cited.  Pyrite, 
marcasite,  galena,  sphalerite,  and  molybdenite  are  found  in  later  vein- 
lets,  often  cutting  the  main  ore-bodies  and  accompanied  by  quartz  and 
calcite. 

MICROSCOPIC  DESCRIPTIONS 

We  are  greatly  indebted  to  Messrs.  F.  L.  Hess,  J.  F.  Kemp,  C.  W. 
Knight,  R.  D.  Longyear,  F.  H.  Mason,  M.  E.  Morgan,  H.  Ries,  and  T. 
L.  Walker  for  specimens  of  rocks  and  ores  from  various  mines  of  the 
Sudbury  district.  Our  study  has  been  facilitated  by  the  excellent  suite  of 
Sudbury  rocks  obtained  from  the  Royal  Ontario  Museum  of  Mineralogy. 
Our  specimens  are  believed  to  be  typical  of  the  Sudbury  ores,  as  we 
repeatedly  find  the  structures  and  relations  described  by  other  workers. 

There  are  three  fairly  distinct  rock  types  directly  associated  with 
the  ores  studied  by  us:  (i)  quartz  norite  almost  free  from  sulfids;  (2) 
pyrrhotite  norite  with  fair  amounts  of  the  sulfids  and  with  uralitized 
pyroxene  and  some  hornblende;  and  (3)  a  hornblende-bearing  granitic 
rock  with  abundant  sulfids  constituting  "the  rich  ore."  These  three 
types  represent  in  a  general  way  the  three  kinds  of  material  found  at 
the  mines:  (i)  the  "lean  ore"  or  the  barren  norite,  (2)  the  pyrrhotite 
norite  transitional  to  the  ore,  and  (3)  the  massive  ore. 

Lean  Ore  from  the  Stobie  Mine. — One  of  the  lean  ores  studied  in  de- 
tail by  us  is  a  rather  fine-grained  quartz  norite,  consisting  of  hypersthene, 
plagioclase,  quartz,  subordinate  biotite  and  hornblende,  magnetite,  pyrrho- 
tite, chalcopyrite,  and  smaller  amounts  of  secondary  chlorite  and  tremo- 
lite.  The  general  relations  are  shown  in  fig.  n  (plate  II). 

The  magnetite  occurs  in  euhedral,  subhedral,  and  anhedral  crystals. 
Most  petrographers  would  assign  the  magnetite  to  the  first  period  of 
crystallization,  but  there  is  clear  evidence  that  the  anhedral  magnetite 


30  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

was  formed  later  than  the  silicates.  This  is  shown  clearly  in  fig.  12. 
The  magnetite  has  completely  surrounded  one  hypersthene  crystal  and 
partly  surrounded  two  others.  All  of  the  magnetite  belongs  to  one 
generation,  for  there  is  a  perfect  gradation  from  the  euhedral  to  an- 
hedral  forms.  Thus  we  have  evidence  that  the  euhedral  magnetite  was 
formed  during  the  late  magmatic  stage.  The  sulfids,  pyrrhotite  and 
chalcopyrite,  which  are  found  in  occasional  spots,  are  also  later  than 
the  silicates,  for  with  the  magnetite  they  form  hook-shaped  anhedra 
surrounding  the  silicates,  as  shown  in  fig.  12.  The  ore-minerals  occur 
between  the  silicate  anhedra,  and  while  they  often  surround  the  sili- 
cates they  rarely  cut  across  an  individual  crystal.  Careful  search,  how- 
ever, usually  reveals  a  few  occurrences  of  this  sort. 

Alteration  products  occur  in  the  sections  figured,  and  the  natural 
inference  would  be  that  the  sulfids  are  connected  in  some  way  with  the 
alteration.  Alteration  of  the  hypersthene  to  tremolite  and  the  formation 
of  chlorite  have  taken  place,  but  these  minerals  have  been  formed  after 
the  introduction  of  the  sulfids,  as  figs.  13  and  14  prove.  In  fig.  13  a 
chlorite  veinlet  cuts  pyrrhotite  and  chalcopyrite,  and  in  the  polished 
section  of  fig.  15  similar  relations  are  shown.  A  veinlet  of  sulfids  to 
the  left  of  the  chlorite  veinlet  of  fig.  13  is  accompanied  by  tremolite, 
but,  as  shown  in  figs.  27,  28,  29  and  30  (plate  VI),  the  sulfid  veinlets 
have  no  connection  with  alteration  products.  The  hypersthene  in  the 
lower  part  of  fig.  12  is  partially  altered,  but  it  shows  exactly  the  same 
relation  to  the  ore-minerals  as  does  the  unaltered  hypersthene  in  the 
upper  part  of  the  figure.  Fig.  14  furnishes  evidence  that  the  tremolite 
is  later  than  the  ore-minerals.  The  sharp  needle-crystals  are  not  residual, 
but  project  out  from  the  altered  hypersthene  into  the  chalcopyrite  mass. 

The  quartz  norite  lean  ore  from  the  Stobie  mine  affords  clear  evi- 
dence that  the  ore-minerals  were  formed  at  the  end  of  the  magmatic 
stage,  that  the  slight  alteration  of  the  hypersthene  took  place  after  the 
introduction  of  the  ore-minerals,  and  that  none  of  the  sulfids  have  under- 
gone rearrangement  of  any  kind. 

Pyrrhotite  Norite  from  the  Stobie  Mine. — We  have  studied  a  typical 
specimen  of  pyrrhotite  norite  from  the  Stobie  mine.  This  rock  consti- 
tutes a  medium  grade  ore  with  large  crystals  of  plagioclase,  aggregates 
of  tremolite,  rims  of  hornblende,  and  sulfid  masses.  The  general  rela- 
tions are  shown  in  fig.  16.  The  light  gray  areas  represented  in  the  pho- 
tograph are  largely  aggregates  of  uralite  needles  in  more  or  less  parallel 
position.  These  aggregates  are  often  surrounded  by  hornblende.  The 
probable  explanation  is  that  hypersthene  was  bordered  by  rims  of  horn- 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  31 

blende,  and  at  a  later  stage  the  hypersthene,  but  not  the  hornblende,  un- 
derwent uralitization.  From  other  data  we  know  that  the  sulfids  were 
formed  after  hypersthene  and  hornblende.  A  study  of  polished  sections 
proves  that  the  uralitization  (tremolitization)  occurred  after  the  intro- 
duction of  the  sulfids,  and  as  evidence  we  introduce  figs.  17  and  18. 

Fig.  1 8  shows  pentlandite  of  the  first  generation  in  veinlike  areas  in 
the  pyrrhotite,  and  pentlandite  of  the  second  generation  developing  along 
crystallographic  directions  of  the  pyrrhotite.  The  rearrangement  of  the 
ores  is  a  very  minor  feature  in  this,  as  in  the  other  Sudbury  specimens. 

The  Rich  Ores. — The  massive  ores  examined  by  us  show  residual 
spots  of  rather  "acid"  material,  consisting  largely  of  alkali  feldspar, 
quartz,  hornblende,  and  biotite.  In  fact  the  gangue  of  the  rich  ore  is 
more  like  the  so-called  micropegmatite  than  the  quartz  norite.  Micro- 
cline  is  abundant,  and  is  often  intergrown  with  quartz.  In  none  of  the 
rich  ores  have  we  been  able  to  find  hypersthene.  Its  place  seems  to 
be  taken  by  hornblende  and  biotite,  which  are  probably  late  magmatic 
alteration  products.  It  is  difficult  to  believe  that  the  "acid"  rock  is  an 
older  foot-wall  granite.  It  is  more  probably  a  felsic  differentiate  of  the 
same  magma  that  furnished  the  norite. 

Rich  Ore  from  the  Stobie  Mine. — Rich  ore  from  the  Stobie  mine 
is  represented  by  fig.  19  (polished  section).  The  chief  minerals  are 
plagioclase,  quartz,  and  biotite.  Garnet  is  also  present.  The  ore-min- 
erals, which  occur  in  irregular  masses  occasionally  extending  out  into 
veinlets,  replace  all  the  silicates  including  garnet.  The  replacement  along 
cleavage  lines  of  biotite  by  the  chalcopyrite  is  beautifully  shown  in 
thin  sections.  This  specimen  is  almost  entirely  free  from  alteration 
products.  There  is,  however,  a  little  chlorite  in  small  lath-shaped  sec- 
tions, and  these  distinctly  cut  the  ores. 

Rich  Ore  from  the  Copper  Cliff  Mine.— On  plate  I  (fig.  7)  we 
show  a  large  polished  hand-specimen  from  this  mine.  This  specimen 
shows  silicates  which  are  believed  to  be  residual.  The  residual  spots 
contain  microcline  and  quartz  (often  in  graphic  intergrowth),  plagio- 
clase, biotite,  a  little  hornblende,  and  long  acicular  crystals  of  apatite. 
As  far  as  can  be  told  from  the  residual  matter  which  has  escaped  re- 
placement, the  rock  is  a  granite.  The  ore-minerals,  magnetite  and 
pyrrhotite,  replace  the  silicates,  especially  biotite,  as  is  shown  in  fig.  20. 
The  biotite  gives  one  the  impression  of  being  one  of  the  last  formed 
silicates.  There  are  also  very  small  biotite  crystals,  which  may  possibly 
belong  to  a  second  generation.  The  apatite  is  also  formed  at  a  late 
stage. 


32  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

There  are  a  few  alteration  products  present,  such  as  chlorite  and 
sericite,  but  they  have  nothing  to  do  with  the  ores. 

Rich  Ores  from  the  Creighton  Mine. — The  Creighton  is  the  largest 
mine  in  the  Sudbury  district,  and  fortunately  our  suite  of  specimens  in- 
cludes a  number  from  that  deposit.  The  residual  silicates  in  the  ores 
are  plagioclase,  microcline,  hornblende,  and  biotite.  Hypersthene  is 
lacking.  Quartz  is  abundant  as  an  interstitial  mineral,  and  often  occurs 
as  a  quartz- feldspar  intergrowth. 

On  account  of  the  abundance  of  quartz,  one  of  the  rich  ores  is  prac- 
tically a  granite  or  possibly  a  grano-diorite.  Another  specimen  very 
much  resembles  the  quartz  norite  in  structure,  but  hornblende  is  present 
instead  of  hypersthene.  It  is  probable  that  this  rock  was  originally  a 
norite,  and  this  indicates  the  possibility  not  only  of  late  magmatic  min- 
erals but  also  of  late  magmatic  rocks.  The  same  specimen  that  fur- 
nished the  thin  section  showing  the  norite  texture  also  shows  micro- 
cline and  a  subgraphic  intergrowth  of  quartz  and  feldspar.  This  varia- 
tion in  the  constituents  within  a  small  space  is  a  characteristic  of  the 
igneous  rocks  containing  magmatic  ores. 

A  photograph  of  rich  ore  from  the  Creighton  mine  is  shown  in  fig. 
8  (plate  I).  This  specimen  furnished  us  the  photomicrographs  of  figs. 
25,  27-30,  32,  and  33.  A  similar  specimen  furnished  photomicrographs 
23,  24,  31,  and  34. 

Polished  sections  showing  the  general  relation  of  the  sulfids  to  the 
silicates  in  these  specimens  are  shown  in  figs.  27,  28,  31,  and  32.  Thin 
sections,  which  are  necessary  for  the  identification  of  the  silicates,  are 
represented  by  figs.  23  and  25. 

The  sulfids  are  later  than  the  silicates.  This  is  almost  certain  from 
the  irregular  hook-shaped  anhedra  which  surround  and  occasionally  pro- 
ject into  the  silicates;  but  if  any  doubt  exists  as  to  the  later  origin  of 
the  sulfids  note  the  veinlets  in  plates  V,  VI,  and  VII.  The  veinlets  replace 
almost  indiscriminately  the  various  silicates  (see  figs.  24,  28,  and  30), 
but  certain  sulfid  areas  show  marked  selective  replacement  of  the  feld- 
spar of  the  quartz- feldspar  intergrowth,  as  in  fig.  31.  For  an  especi- 
ally good  illustration  of  replacement  veinlet  see  fig.  24,  where  fresh 
biotite  has  been  invaded  by  the  sulfid.  In  fig.  26  a  hornblende  crystal 
has  been  cut  squarely  in  two  by  a  sulfid  veinlet.  Note  the  branch- 
ing veinlets  of  figs.  28  and  30.  The  only  explanation  of  these  veinlets 
is  that  they  are  later  than  the  silicates.  The  proponents  of  the  magmatic 
theory  for  the  Sudbury  ores  have  ascribed  these  veinlets  to  rearrange- 
ment, and  thus  they  reconcile  their  theories  with  microscopic  work  such 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  33 

as  that  of  Dickson.  That  the  sulfid  veinlets  are  of  the  same  generation 
as  the  main  sulfid  masses  and  are  not  due  to  later  rearrangement  is 
shown  in  fig.  29,  where  the  sulfids  extend  out  into  a  veinlet  without  any 
break  in  the  continuity  or  other  evidence  of  later  origin. 

The  late  origin  of  the  sulfid  veinlets  can  hardly  be  doubted,  but 
some  may  be  inclined  to  suggest  a  hydrothermal  origin  for  them.  This 
is  disproved  by  our  findings.  For  example  in  the  thin  sections  of  the 
Creighton  ore  we  found  tremolite  (and  possibly  talc)  pseudomorphous 
after  original  hypersthene.  The  pseudomorphs  contain  minute  magne- 
tite crystals  which  were  formed  by  alteration.  The  pseudomorphs  are 
cut  by  veinlets  of  chalcopyrite,  but  the  chalcopyrite  has  not  wandered 
into  the  cracks  of  the  tremolite.  This  shows  that  the  hydrothermal 
alteration  of  hypersthene  to  tremolite  (or  talc)  and  magnetite  occurred 
after  the  introduction  of  the  chalcopyrite. 

That  the  magnetite,  as  well  as  the  sulfids,  is  formed  at  a  late  mag- 
matic  stage  is  indicated  by  fig.  20.  The  magnetite  contains  included 
ilmenite  plates  as  illustrated  by  fig.  21.  This  magnetite-ilmenite  inter- 
growth  is  a  characteristic  feature  of  magmatic  ores. 

Polished  sections  of  massive,  almost  solid,  ore  from  the  Creighton 
mine  are  represented  by  figs.  22,  35,  and  36.  Fig.  22  shows  an  area  of 
silicates  which  is  doubtless  a  relict  of  the  same  granitic  material  present 
in  the  other  Creighton  samples.  Magnetite  has  been  replaced  by  the 
sulfids  and  the  pentlandite  has  replaced  pyrrhotite  in  vein-like  masses. 
Veins  of  pentlandite  are  well  shown  in  fig.  35 ;  also  minute  tufts  of 
brush-like  crystals  of  a  pale-yellow  mineral,  probably  pentlandite  of  a 
second  generation.  Fig.  36  is  a  highly-magnified  view  of  one  of  these 
crystals  which  extends  out  from  a  veinlet  of  chalcopyrite,  probably  of 
the  second  generation.  Figs.  35  and  18  show  what  a  minor  amount  of  re- 
arrangement has  taken  place  in  the  typical  magmatic  ore  of  Sudbury. 

At  some  of  the  Sudbury  mines,  notably  the  Worthington,  certain 
sulfids  which  are  not  typically  magmatic  have  been  found.  Among 
these  minerals  are  pyrite  and  polydymite,  but  many  others  have  been 
reported.46 

We  have  examined  several  Sudbury  ores  containing  pyrite.  In 
fig.  37  is  represented  a  supposed  specimen  of  polydymite  from  the  Ver- 
milion mine.  This  contains  pyrite  in  the  form  of  veinlets  evidently 
formed  at  a  late  stage.  The  relation  of  the  polydymite  to  the  magmatic 
sulfids  is  not  entirely  certain,  but  the  examination  of  a  very  fresh  speci- 

46  Barlow,  A.  E.— The  nickel  and  copper  deposits  of  Sudbury,  Ontario.  Geol. 
Surv.  Can.  Ann.  Rept.  14,  pt.  H,  93  et  seq. 


34  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

men  suggests  that  the  Sudbury  polydymite  is  a  mixture  of  three  min- 
erals: pentlandite,  an  unknown  violet-gray  mineral,  and  the  true  polyd- 
ymite. The  polydymite  and  violet-gray  mineral  are  probably  due  to  the 
breaking  down  of  the  pentlandite. 

One  of  the  later  sulfid  ores  from  the  Worthington  mine  is  shown 
in  fig.  38.  Pyrite  and  sphalerite  occur  in  a  gangue  of  calcite.  The  pyrite 
has  a  peculiar  reticulate  structure.  The  sphalerite  is  later  than  pyrite. 

The  order  of  succession  of  the  ore-minerals  at  Sudbury  as  deter- 
mined in  polished  sections  is  as  follows:  (i)  magnetite,  (2)  pyrrhotite, 
(3)  pentlandite,  and  (4)  chalcopyrite.  The  other  sulfids,  such  as  pyrite, 
sphalerite,  etc.,  are  post-magmatic. 

ORIGIN  OF  THE  SUDBURY  ORES 

With  the  possible  exception  of  the  gold-bearing  "banket"  of  the 
Rand  (South  Africa),  perhaps  no  other  single  group  of  ore  deposits 
has  received  as  much  attention  from  the  standpoint  of  origin  as  have 
the  Sudbury  deposits.  Both  the  igneous  and  hydrothermal  hypotheses  of 
origin  have  had  an  almost  equal  number  of  advocates.  Altho  the  earliest 
papers  on  the  Sudbury  deposits  advocated  the  aqueous  origin  of  the  ores, 
opinion  has  been  gradually  crystallizing  in  favor  of  the  magmatic  origin, 
largely  on  account  of  the  studies  of  Barlow,  Coleman,  and  Walker,  not- 
withstanding several  vigorous  protests,  notably  that  of  Dickson.  How- 
ever, the  latest  paper47  on  the  subject  reopens  the  whole  question. 

Our  work  substantiates  many  of  the  findings  of  Dickson  48  and  of 
Campbell  and  Knight 49  relative  to  the  Sudbury  ores.  We  find,  as  they 
did,  that  the  sulfids  were  formed  later  than  the  silicates,  and  verify 
Dickson's  conclusion  that  the  amount  of  hornblende  increases  as  the  ores 
become  richer.  With  the  exception  that  magnetite  is  later,  not  earlier, 
than  the  silicates,  we  agree  with  Campbell  and  Knight  as  to  the  order 
of  formation  of  the  ore-minerals. 

We  disagree  with  Dickson  as  to  the  hydrothermal  ("secondary 
aqueous")  origin  of  the  ores. 

We  agree,  on  the  other  hand,  with  the  supporters  of  the  magmatic 
hypothesis  that  the  ores  were  formed  within  the  magmatic  period.  They 
were,  however,  not  formed  at  an  early  stage  and  not  by  the  sinking  of 
the  sulfid  constituents. 


"Knight,  C  W.— Loc.  cit. 

48  Dickson,  C.  W. — The  ore-deposits  at  Sudbury,  Ontario.     Trans.  Am.  Inst. 
Min.  Eng.,  34,  3-67  (1903). 

49  Campbell,  W.,  and  Knight,  C.  W. — On  the  microstructure  of  nickeliferous 
pyrrhotite.     Econ.  Geol.,  2,  350-356  (1907). 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  35 

In  fine,  our  work  reconciles  the  almost  diametrically  opposite  views 
of  these  two  groups  of  investigators.  Altho  the  ores  are  believed  to  be 
magmatic,  they  have  been  formed  at  the  end  of  the  magmatic  period 
by  the  replacement  of  the  silicates. 

BIBLIOGRAPHY  OF  THE  SUDBURY  ORE  DEPOSITS 

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Feb.  1894. 
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Ann.  Rept.  14,  pt.  H,  236  pp.  (1904). 

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Geol.  Soc.  Can.,  2,  125-137  (1891). 
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Can.  Geol.   Surv.  Reports,  Report  F  (1893). 
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56,  565-566   (1893). 

— Segregation  in  ores  and  mattes.     Can.  Rec.  Sci.,  7,  176-190   (1896). 
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pyrrhotite.    Econ.  Geol.,  2,  350-366  (1907)- 
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pp.  235-299- 

The  northern  nickel  range.     Ont.  Bur.  Mines,  Rept.   1904,  pt.  i,  192-222. 

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Geology  of  the  Sudbury  district.     Eng.  and  Min.  Jour.,  79,  189-190  (1905). 

The  Sudbury  nickel  field.     Ont.  Bur.  Mines,  Rept.  1905,  14,  pt.  3,  188  pp. 

Magmatic   segregation   of   sulphide   ores.     Abstract,   British  Assn.   Adv.    Sci., 

Rept.  75th  Meeting,  400  (1906). 

The   Sudbury   laccolithic   sheet.     Jour.   Geol.,    15,   759-782    (1907). 

Die  Sudbury  Nickelerze.    Zeit.  f.  prakt.  Geol.,  15,  221  (1907). 

The  Sudbury  nickel  ores.     Geol.  Mag.,  5,  18-19   (1908). 

Copper  and  nickel  deposits  of  Canada.    Abstract,  British  Assn.  Adv.  Sci.,  Rept. 

79th  Meeting,  479-480   (1910). 
The  nickel   industry,  with  special  reference  to  the  Sudbury  region,   Ontario. 

Can.  Dept.  Mines,  Mines  Branch.    Publ.  170,  260  pp.  (1912). 


36  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

—The  Sudbury  area.     I2th  Inter.  Geol.  Cong.     Guide-Book  no.  7,  48  pp.   (1913). 
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(1913)- 
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Econ.  Geol.,  10,  39O-393   (1915)- 
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Sudbury.     Eng.  and  Min.  Jour.,  73,  660  (1902). 
Note  on  the  condition   of  platinum   in  the  nickel-copper  ores   from   Sudbury. 

Am.  Jour.  Sci.,  (4)  15,  137-139  (1903). 
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(1909). 
Gamier,  J. — Mines  de  Nickel,  Cuivre,  et  Platine  du  District  de  Sudbury,  Canada. 

Mem.  Soc.  des  Ing.  Civiles,  Paris,   1891. 
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139-140  (1908). 
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335  (1905). 
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223-235   (1906). 

The  Sudbury  nickel  region.     Eng.  and  Min.  Jour.,  82,  313-314   (1906). 

Hore,  R.  E. — Origin  of  the  Sudbury  nickel  and  copper  deposits.     Min.  and  Eng. 

World,  36,  1345-1349  (1912). 
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Surv.,  Pub.  16,  pp.  11-37  (1914)- 
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505-522  (1914). 

Kemp,  J.  F. — An  outline  of  views  held  today  on  the  origin  of  ores.     Mineral  In- 
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101,  811-812  (1916). 
Miller,  W.   G. — On   some  nickeliferous  magnetites.     Brit.   Assn.  Adv.  Sci.,   Rept. 

1897,  pp.  660-661   (1898). 
Miller,  W.  G.,  and  Knight,  C.  W.— Sudbury,  Cobalt,  and  Porcupine  geology.    Eng. 

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upon   three   supposed  new   species   from  the   same   region.     Am.   Jour.    Sci., 

(3)  45,  493-494  (1893). 
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Inst.,  5,  528-551    (1902). 
St.   Clair,   S.— Origin  of  the   Sudbury  ore   deposits.     Min.   and    Sci.   Press,   109, 

243-246  (1914)- 
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Ontario.    Jour.  Can.  Min.  Inst,  11,  567-585  (1908). 
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(1907). 
Stutzer,    O. — Die    Nickelerzlagerstatten   bei    Sudbury   im   Kanada.     Zeit.   f.   prakt. 

Geol.,  16,  285-287  (1908). 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS  37 

Thomas,  K.— The  Sudbury  nickel  district,  Ontario,  Canada.     Min.  and  Sci.  Press, 

105,  433  (1912). 
—The    Sudbury   nickel   district   of   Ontario.      Eng.   and   Min.   Jour.,   97,    152-154 

(1914)- 

Thompson,  P.— The  Sudbury  nickel  region.     Eng.  and  Min.  Jour.,  82,  3-4  (1906). 
von  Foullon,  H.  B.— t)ber  einige  Nickelerzvorkommen.    Jahr.  d.  k.k.  geol.     Reichs- 

anstalt,  42,  223-310.    Vienna  (1892). 
Walker,   T.    L—  Notes   on   nickelifercus   pyrite   from  the   Murray   mine,    Sudbury, 

Ont.    Am.  Jour.  Sci.,  (3)  47,  312-314  (1899). 
— Geological  and  petrographical  studies  of  the   Sudbury  nickel   district.     Quar. 

Jour.  Geol.  Soc.,  53,  40-66  (1897). 
—Certain  mineral  occurrences  in  the  Worthington  mine,  Sudbury,  Ontario,  and 

their  significance.     Econ.  Geol.,    10,  536-543    (1915). 

Williams,  G.  H.— Notes  on  the  microscopical  character  of  rocks  from  the  Sudbury 
mining  district.  Can.  Geol.  Surv.  Reports,  (new  series)  5,  pt.  I,  Report  F 
(1893). 

THE  ALEXO  MINE,  ONTARIO,  CANADA 
GEOLOGY 

This  occurrence  of  nickeliferous  pyrrhotite,  twenty  miles  southeast 
of  Porcupine,  Ontario,  has  been  mentioned  briefly  by  Coleman,50  and 
described  in  detail  by  Uglow.51  The  geological  relations  are  not  dis- 
closed on  account  of  lack  of  exposure,  and  the  mineralogical  relations 
are  complicated  by  an  intense  serpentinization.  The  ore  occurs  as  a  lens 
in  serpentinized  rock  at  the  contact  with  rhyolite.  In  cases  of  this  kind 
microscopic  work  is  especially  valuable.  The  unaltered  rock  is  consid- 
ered by  Uglow  to  have  been  a  peridotite  of  the  wehrlite  or  harzburgite 
variety.  The  only  exposure  is  in  the  immediate  vicinity  of  the  ore  body, 
and  developments  are  not  sufficient  to  show  the  structural  relations. 

UGLOW'S  CONCLUSIONS 

Uglow  examined  the  ore  in  both  thin  and  polished  sections,  has 
shown  the  relations  in  photographs,  and  has  brought  out  in  a  convincing 
way  the  phenomena  of  replacement  as  shown  by  his  microscopic  study. 
He  states  that  the  ore  (pyrrhotite,  pentlandite,  chalcopyrite)  "eats  its 
way  through  the  matrix  of  the  serpentine"  .  .  .  "replaces  it,  forming 
a  network  of  ore"  .  .  .  "extends  between  the  crystals  into  fractures 
and  cracks  in  the  latter"  .  .  .  that  "ore  replaces  part  or  all  of  an 
olivine  crystal"  and  produces  pseudomorphs — "magnetite  pseudomorphs 


50  Coleman,  A.  P.— The  Alexo  nickel  deposit.     Jour.  Geol.,  5,  373-376  (1910). 

51  Uglow,  W.  L.— A  new  nickel  occurrence  in  northern  Ontario.     Jour.  Can. 
Min.  Inst,  14,  657-677  (1911). 


38  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

which  have  resulted  from  the  alteration  of  the  olivine,  have  become  partly 
or  wholly  replaced  by  pyrrhotite,"  etc. 

He  finds  that  the  order  of  the  formation  of  the  ore-minerals  is  (i) 
magnetite,  (2)  pyrrhotite,  (3)  pentlandite  and  chalcopyrite ;  and  re- 
marks: "It  is  difficult  to  conceive  of  the  sulphides  as  differentiations 
from  a  molten  magma  when  they  are  as  a  matter  of  fact  deposited  one 
after  the  other,  the  younger  ones  occurring  as  vein-like  masses  in  the 
older." 

His  work  is  incomplete  in  that  he  does  not  investigate  what  por- 
tion of  the  phenomena  of  replacement  and  ore  migration  is  to  be  con- 
nected with  serpentinization,  and  what  portion  with  the  first  deposition 
of  the  ores  prior  to  serpentinization.  Lindgren  *2  states :  "Here,  as  in 
so  many  other  cases,  secondary  changes  seem  to  have  been  confused 
with  primary  deposition." 

We  have,  therefore,  investigated  in  detail  the  relative  amount  of 
transfer  and  migration  that  is  connected  with  serpentinization,  and  have 
been  able  to  show  that  the  phenomena  of  replacement  mentioned  above 
antedate  serpentinization,  and  are  connected  with  the  original  deposition 
of  the  ores. 

MICROSCOPIC  DESCRIPTION 

Polished  sections  of  ore  of  the  Alexo  mine  kindly  furnished  by  Mr. 
F.  H.  Mason  of  the  Canadian  Government  Exhibition  Commission,  show 
pyrrhotite  with  minor  amounts  of  pentlandite  and  chalcopyrite  in  a 
gangue  of  serpentine.  The  serpentine  is  the  result  of  alteration  of  oli- 
vine. The  general  relations  of  the  minerals  are  shown  in  fig.  39.  Resid- 
ual cores,  apparently  of  olivine,  prove  to  be  serpentine  upon  examination 
of  the  thin  sections.  See  also  fig.  40,  which  is  an  enlargement  of  a  por- 
tion of  fig,  39.  The  section  shown  in  figs.  39  and  40  was  polished  to 
bring  out  the  silicates.  On  the  other  hand,  the  surface  shown  in  figs.  41-44 
was  polished  especially  for  the  sulfids.  Pentlandite  occurs  in  fair  amounts 
rather  evenly  distributed  through  the  ore,  with  a  tendency  to  alignment 
along  crystallographic  directions  of  the  pyrrhotite  which  it  replaces.  The 
other  sulfid  present  is  chalcopyrite,  which  occurs  in  two,  or  possibly 
three,  generations.  In  fig.  41  there  is  a  little  magmatic  chalcopyrite  and 
pentlandite.  The  serpentinization  has  been  accompanied  by  a  migration 
of  nickel  to  form  a  second  generation  of  pentlandite  along  veinlets  (fig. 
42),  and  of  copper  to  form  a  second  generation  of  chalcopyrite  (fig.  44). 
The  minute  euhedral  crystals  of  chalcopyrite  shown  in  fig.  44  may  pos- 

"  Lindgren,  W.— Mineral  Deposits,  765. 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS  39 

sibly  represent  a  third  generation  of  chalcopyrite.  Altho  there  has  been 
extensive  alteration  of  the  silicates  in  the  Alexo  ore,  microscopic  study 
shows  clearly  (see  especially  fig.  43)  that  this  alteration  was  subsequent 
to  the  main  period  of  ore  formation  and  that  the  migration  of  ore  during 
serpentinization  was  relatively  slight. 

SUMMARY 

Summarizing  the  stages  in  the  Alexo  ore  deposit  we  have  (i)  the 
formation  of  pyrrhotite,  pentlandite,  and  chalcopyrite  in  the  order  named, 
at  the  end  of  the  magmatic  period,  probably  by  selective  replacement  of 
the  ground-mass  of  a  picrite  (pseudo-porphyritic  peridotite)  ;  (2)  the 
alteration  of  olivine  and  possibly  of  other  silicates  to  serpentine;  (3) 
accompanying  the  serpentinization  there  was  a  slight  migration  of  copper 
and  nickel  to  form  second  generations  of  chalcopyrite  and  pentlandite 
respectively,  and  also  the  formation  of  magnetite  in  very  minute  crystals 
and  veinlets. 

BIBLIOGRAPHY  OF  THE  ALEXO  ORE  DEPOSITS 

Coleman,  A.  P. — The  Alexo  nickel  deposit.     Econ.  Geol.,  5,  373-376  (1910). 
The  nickel  industry  with  special  reference  to  the  Sudbury  region,  Ontario. 

Can.  Dept.   Mines,  Mines  Branch.     Report   170,  pp.   112-114   (1912). 
Uglow,  W.  L. — The  Alexo  nickel  deposit,  Ontario.     Ont.  Bur.  Mines.     Ann.  Rept. 

20,  pt.  2,  34-39  (1911). 
The  Alexo  mine.     A  new  nickel  occurrence  in  northern  Canada.     Jour.  Can. 

Min.  Inst.,  14,  657-677  (1911). 
Summary  report  on  the  Sudbury  nickel  field.    Can.  Dept.  Mines,  Mines  Branch, 

Summary  Rept.  1911,  pp.  87-89. 

THE  FRIDAY  MINE,  SAN  DIEGO  COUNTY,  CALIFORNIA 
The   occurrence   of   nickel   ores    in   the   Friday   mine,    San   Diego 
county,  California,  was  reported  by  Merrill  62°  and  described  by  Calk- 
ins,626 whose  paper  furnished  the  geological  data  summarized  in  the  fol- 
lowing paragraphs : 

The  mine  is  four  miles  from  the  town  of  Julian,  which  lies  half- 
way between  San  Diego  and  the  Salton  Sea.  It  is  located  on  the  crest  of 
the  broad  range  which  forms  the  northern  extension  of  the  Cordillera  of 
Lower  California. 


62«  Merrill,  F.  J.  H. — Geology  and  Mineral  Resources  of  San  Diego  and  Im- 
perial  counties.      California   State   Mining   Bureau    Report,    biennial   period    1913- 

1914,  P-  40. 

626  Calkins,  F.  C— An  occurrence  of  nickel  ore  in   San  Diego  county,  Cali- 
fornia.   S.  Bull.  640,  U.  S.  Geol.  Surv.,  77-82  (1916). 


40  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

The  complex  batholith  of  Lower  California  extends  into  San  Diego 
county,  and  the  granitic  rocks  of  the  region  are  probably  closely  related 
to  the  larger  masses  to  the  south.  Metamorphosed  sediments  are  included 
within  the  granite,  and  near  the  mine  a  mass  of  gabbro  of  unknown  size 
lies  in  contact  with  mica  schist. 

The  ore  occurs  as  a  shoot  or  lens  at  the  contact  of  the  gabbro  with 
the  schist.  The  latter  dips  steeply  southward,  and  the  ore  lies  at  the 
base  of  the  gabbro.  Considerable  fracturing  is  reported  in  the  vicinity 
of  the  ore  body,  and  the  ore  is  penetrated  by  a  small  pegmatite  dike 
containing  conspicuous  crystals  of  tourmaline. 

MICROSCOPIC  DESCRIPTION 

Thru  the  kindness  of  Mr.  Beecher  Sterne,  president  of  the 
Friday  Copper  Mines  Company,  we  obtained  a  suite  of  rocks  and 
ores  from  the  Friday  mine,  including  specimens  from  the  recently 
developed  lower  levels.  The  rock  is  an  olivine  gabbro  with  plagioclase 
(Ab3  An7  in  one  section),  olivine,  and  both  orthorhombic  and  monoclinic 
pyroxene.  A  pale  brown  hornblende  occurs  as  rims  around  the  border 
of,  and  as  patches  within,  the  pyroxene.  The  hornblende  is  in  parallel 
position  with  the  pyroxene,  and  is  doubtless  a  magmatic  alteration 
product.  Small  tremolite  prisms  replace  the  pyroxene  and  plagioclase. 
Another  alteration  product  is  calcite,  which  occurs  in  veinlets  and  occa- 
sionally in  zonal  crystals. 

The  polished  sections  of  the  massive  ores  consist  largely  of  pyrrho- 
tite  with  residual  spots  of  silicates.  Associated  with  the  pyrrhotite  is  a 
considerable  amount  of  pentlandite.  Calkins  reported  this  as  polyd- 
ymite,  but  it  has  the  characteristic  cleavage,  relief,  and  color  of  pent- 
landite. Chalcopyrite  in  small  amounts  is  also  present  in  the  sections,  and 
is  distinctly  later  than  the  pyrrhotite  and  pentlandite.  A  second  genera- 
tion of  pentlandite  is  developed  along  cracks  in  the  pyrrhotite.  A  brass- 
yellow  mineral  occurs  in  veinlets  and  reticulate  masses  as  a  replacement 
of  pyrrhotite.  This  was  called  pyrite  by  Calkins,  but  more  probably  it 
is  marcasite.  The  marcasite  gives  the  impression  of  being  a  very  late 
mineral.  It  is  usually  extensively  developed  along  calcite  veinlets  which 
occur  as  a  net  of  intersecting  stringers  cutting  all  the  other  minerals. 
The  polished  sections  also  show  that  tremolite  is  later  than  the  sulfids. 

SUMMARY 

In  the  Friday  deposit  we  have  the  following  sequence  of  events: 
(i)  The  crystallization  of  olivine,  pyroxenes,  and  plagioclase.  (2)  A 
slight  development  of  hornblende  by  magmatic  alteration.  (3)  The 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS  41 

formation  of  pyrrhotite,  pentlandite,  and  chalcoyprite  in  the  order 
named  by  the  replacement  of  the  above  mentioned  silicates.  (4)  The 
development  of  tremolite  as  a  hydrothermal  mineral  and  the  development 
of  pentlandite  of  a  second  generation  in  cracks  in  pyrrhotite.  (5)  The 
extensive  development  of  calcite  and  marcasite  veinlets. 

With  the  exception  of  the  marcasite  and  calcite  this  deposit  is 
similar  in  mineral  composition  and  paragenesis  to  magmatic  deposits  of 
this  type. 

THE  GOLDEN  CURRY  MINE,  ELKHORN,  MONTANA" 
GEOLOGY 

An  interesting  deposit  of  magmatic  gold-  and  copper-bearing  pyrrho- 
tite (non-nickeliferous,  however)  occurs  in  a  marginal  "basic"  segre- 
gation of  the  Boulder  quartz  monzonite  batholith.  The  deposit  thus 
differs  from  the  normal  type  of  magmatic  pyrrhotite  deposits  in  respect 
to  the  geological  relations  and  the  character  of  the  mother  rock.  The 
mineral  composition,  the  inclusion  of  the  ore  in  a  subsilicic  rock,  and  the 
relatively  unaltered  condition  of  the  ore  and  country  rock,  give  this 
deposit  the  characteristics  of  a  magmatic  ore.  It  is  possibly  the  unmeta- 
morphosed  equivalent  of  the  lenses  of  pyrrhotite  in  "basic"  layers  of 
gneiss,  and  is  an  indication  that  certain  of  these  puzzling  occurrences  are 
altered  magmatic  segregations. 

The  Boulder  batholith  is  well  known  to  students  of  ore  deposits 
because  it  was  accompanied  and  followed  by  extensive  mineralization, 
and  especially  because  it  encloses  the  enormous  copper  deposits  at  Butte. 

The  Golden  Curry  mine  at  Elkhorn  is  in  a  region  of  intense  min- 
eralization,54 which  occurred  in  and  near  the  roof  of  the  batholith.  In 
this  property  there  are  two  types  of  ore  deposits:  (i)  a  contact 
deposit  between  quartz  monzonite  and  limestone,  consisting  of  magnetite 
and  some  chalcopyrite  accompanied  by  garnet;  (2)  the  magmatic  pyrrho- 
tite deposit.  The  latter  occurs  250  feet  from  the  contact  mentioned 
above,  as  the  sulfid  rich  portion  of  a  lens  of  fresh  monoclinic  pyroxene, 
pyrrhotite,  and  chalcopyrite.  This  is  surrounded  by  a  border  zone,  which 
grades  into  the  normal  quartz  monzonite  by  the  addition  of,  first,  pla- 
gioclase,  then  hornblende,  quartz,  and  the  accessory  minerals  of  the 
quartz  monzonite,  and  finally  biotite  and  orthoclase.  According  to 


53  Knopf,  A. — A  magmatic  sutphide  ore-body  at  Elkhorn,  Mont.     Econ.  Geol. 

8,  323-336  (1913)- 

54  Knopf,   A. — Ore   deposits   of   the   Helena  mining   region,   Montana.     Bull. 

527,  U.  S.  Geol.  Surv.  (1913). 


42  A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 

Knopf,  the  deposit  is  characterized  by  the  absence  of  pneumatolytic  or 
hydrothermal  alteration  products.  In  the  vicinity,  however,  intense 
pneumatolytic  action  is  shown  in  the  Queen  mine,  the  ore  of  which  is 
argentiferous  galena,  accompanied  by  quartz  and  tourmaline.  Strong 
mineralization  of  a  later  cooler  phase  is  also  represented  in  the  highly 
productive  Elkhorn  mine.  The  ore  is  a  replacement  deposit  in  dolomite, 
consisting  of  argentiferous  galena  unaccompanied  by  metasomatic  gangue 
minerals. 

The  Boulder  batholith  develops  in  places  a  border  zone  distinctly 
more  basic  than  the  normal  quartz  monzonite.  The  ore  deposits,  how- 
ever, with  the  exception  of  the  Golden  Curry  mine,  are  connected  with 
the  "acid"  differentiation  products,  as  emphasized  by  Billingsley.55  Aplite 
masses,  aplite  and  pegmatite  dikes,  often  accompanied  by  tourmaline 
and  sulfids,  are  well  developed  in  and  near  the  roof  of  the  batholith. 
Contact  deposits  are  common,  with  axinite,  tourmaline,  etc.,  in  addition 
to  the  usual  minerals.  Tourmaline-copper  and  tourmaline-lead-silver 
ores  are  the  high-temperature  phases  brought  about  by  the  mineralizing 
action  of  the  intrusive.  It  is  therefore  of  considerable  interest  to  know 
whether  the  "basic"  segregation  of  the  Golden  Curry  mine  was  formed 
at  an  early  or  a  late  stage.  Was  there  a  gap  between  the  magmatic 
deposits  and  the  later  high-temperature  phase  so  extensively  developed 
in  this  region? 

MICROSCOPIC  DESCRIPTION 

Mr.  Adolph  Knopf  of  the  United  States  Geological  Survey  has 
kindly  furnished  us  with  thin  sections  of  the  pyrrhotite-augite  ore  from 
the  Golden  Curry  mine.  The  rock  is  practically  a  pyroxenite,  consisting 
mainly  of  monoclinic  pyroxene  (augite)  and  pyrrhotite,  with  minor 
amounts  of  hornblende,  tremolite,  and  chalcopyrite. 

The  replacement  of  pyroxene  by  the  pyrrhotite  and  chalcopyrite  is 
well  shown  in  fig.  45.  The  ore-minerals  not  only  surround  the  pyroxene, 
but  occasionally  cut  straight  across  a  pyroxene  crystal,  as  shown  a  little 
to  the  left  and  below  the  center  of  the  photograph.  The  replacement  of 
the  pyroxene  by  the  sulfids  is  also  proved  by  fig.  47,  for  the  small 
pyroxene  crystal  near  the  center  is  cut  in  two  by  a  veinlet.  The 
pyroxene  has  been  altered  to  green  hornblende  (not  uralite)  in  occa- 
sional patches  (the  darker  gray  spots  in  figs.  45  and  47).  Definite  proof 
that  the  sulfids  are  later  than  the  hornblende  and  replace  it  along 
cleavage  lines  is  shown  in  the  lower  portion  of  fig.  46.  Tremolite  is 


88  Billingsley,  P.— The  Boulder  batholith  of  Montana.     Tras.  Am.  Inst.  Min. 
Eng.,  51,  31-56  (1915)- 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  43 

formed  at  a  still  later  stage,  for  as  shown  in  fig.  47,  it  projects  from  the 
end  of  a  hornblende  crystal  into  the  sulfids. 

We  agree  with  Knopf  that  the  pyrrhotite-chalcopyrite  ore  of  the 
Golden  Curry  mine  is  a  magmatic  deposit  showing  very  little  post- 
mineral  alteration,  but  we  believe  that  the  sulfids  were  formed  at  a  late 
magmatic  stage,  and  after  the  partial  magmatic  alteration  of  pyroxene 
to  hornblende. 


PROSPECT  HILL,  LITCHFIELD,  CONNECTICUT 

An  interesting  series  of  sulfid-bearing  igneous  rocks  varying  from 
gabbro  and  norite  to  peridotite  and  pyroxenite  was  described  by  Hobbs  56 
from  this  locality.  Extreme  magmatic  differentiation  has  produced  a 
great  variety  of  rock  types  within  a  small  area. 

Recently  Howe  57  has  studied  the  relation  of  the  sulfids  to  the  sili- 
cates in  these  rocks.  He  identifies  pyrrhotite,  pentlandite,  and  chal- 
copyrite,  which  were  formed  in  the  order  named.  A  prominent  feature 
is  the  presence  of  hornblende  and  biotite  as  magmatic  alteration  products 
of  pyroxene,  but  no  mention  is  made  of  any  post-magmatic  alteration. 
Howe  concludes  that  the  ore-minerals  mentioned  are  magmatic  sulfids 
which  separated  for  the  most  part  at  an  early  stage  in  the  cooling  of 
the  magma,  but  remained  liquid  until  the  silicates  had  crystallized.  He 
believes,  however,  that  the  sulfid  veinlets  in  the  silicates  were  formed  by 
the  replacement  of  the  latter. 

Dr.  Howe  has  kindly  sent  us  some  specimens  of  these  interesting 
sulfid-bearing  rocks,  and  we  have  corroborated  nearly  all  of  his  findings. 
The  Litchfield  sulfids  are  magmatic  sulfids,  if  such  exist.  There  is  no 
evidence  of  any  rearrangement  of  the  ore-minerals.  The  veinlets 
do  not  belong  to  a  later  generation,  and  all  the  ore-minerals  are  late  mag- 
matic. Altho  the  rocks  are  exceptionally  fresh,  perhaps  more  so  than 
the  average  unmineralized  igneous  rock,  we  find  some  serpentinization  of 
the  olivine.  This  is  especially  well  shown  in  the  polished  sections.68 

We  find  that  there  has  been  slight  post-magmatic  alteration  of  the 
silicates,  but  conclude  with  Howe  59  "that  the  relations  of  the  sulphides 
to  one  another  were  .  .  .  determined  during  the  magmatic  period." 


56  Festschrift  Harry  Rosenbusch,  25-48.     Stuttgart,  1906. 

67  Howe,  E. — Sulphide-bearing  rocks  from  Litchfield,  Conn.     Econ.  Geol.,  10, 

330-347  (IQIS). 

58  The  alteration  products  are  very  apparent  in  well  polished  sections,  tho 
of  course  the  identity  of  the  minerals  must  be  determined  in  thin  sections  or 
crushed  fragments. 

89  Loc.  cit,  339. 


44  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 


KNOX  COUNTY,  MAINE 

Bastin  60  has  described  in  detail  a  pyrrhotitic  peridotite  from  this 
locality.  The  peridotite  is  thought  to  be  a  differentiate  of  granite.  Chal- 
copyrite  and  pyrite  are  associated  with  the  pyrrhotite,  but  the  relation  of 
the  pyrite  to  the  other  ore-minerals  is  not  discussed.  Hornblende  is 
locally  important,  and  occurs  both  as  crystals  and  as  rims  between  feld- 
spar and  olivine.  The  secondary  alteration  (serpentinization)  is  later 
than  the  ore.  His  photographs  show  pyrrhotite  surrounding  the  olivine 
and  penetrating  it  in  embayments. 

This  is  evidently  a  typical  magmatic  deposit,  with  the  ore  later  than 
the  primary  silicates. 

MOUNTAIN,  WISCONSIN" 

This  deposit  occurs  in  a  basic  dike  60  to  200  feet  wide.  The  ore  is 
massive  pyrrhotite  which  "merges  into  gabbro  on  indefinite  lines."  The 
author  does  not  give  the  results  of  a  careful  field  study,  and  no  micro- 
scopic investigation  of  the  rock  or  ore  is  recorded. 

OTHER  PYRRHOTITE  DEPOSITS  IN  THE  UNITED  STATES 
In  addition  to  its  occurrence  in  the  magmatic  ores,  pyrrhotite  is 
also  found  in  other  high-temperature  deposits.  Among  these  are  the 
lenticular  ore-bodies  found  in  gneisses  and  schists.  Those  in  the  basic 
portion  of  the  gneiss  may  be  magmatic  in  origin,  and  those  in  the  acid 
layers  may  be  related  to  "acid"  or  pegmatitic  extracts.  Altho  some  of 
the  pyrrhotite  deposits  occurring  in  metamorphic  rocks,  for  example 
that  of  the  Gap  mine,  Lancaster,  Pennsylvania,62  have  been  assigned  to 
the  magmatic  group,  most  of  them  have  been  so  modified  by  intense 
metamorphism  that  they  no  longer  show  the  characteristics  of  typical 
magmatic  deposits. 

INSIZWA  RANGE,   EAST   GRIQUALAND,   SOUTH  AFRICA 
Our  knowledge  of  the  deposits  near  the  town  of  Mount  Ayliff,  in 
Cape  Colony,  is  due  chiefly  to  the  excellent,  tho  brief,  report  of  Du 


80  Bastin,  E.   S. — A  pyrrhotitic  peridotite   from   Knox  county,  Maine.     Jour. 
Geol.,  16,  124-138  (1908). 

61  Bagg,  R.  M. — The  discovery  of  pyrrhotite,  with  a  discussion  of  its  prob- 
able origin  by  magmatic  differentiation.    Econ.  Geol.,  8,  369-373  (1913). 

62  Kemp,  J.   F. — The   nickel  mine  at   Lancaster   Gap,   Pennsylvania,   and   the 
pyrrhotite  deposits   at  Anthony's   Nose,  on  the   Hudson.     Trans.   Am.   Inst.  Min. 
Eng.,  24,  620-633   (1894). 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS  45 

Toit.63  He  has  discussed  in  a  satisfactory  manner  the  general  geology 
and  the  petrography  of  the  region.  His  microscopic  description  of  the 
ores  is  brief,  and  it  is  chiefly  in  this  respect  that  further  information 
is  needed.  Unfortunately  we  have  not  been  able  to  procure  specimens 
from  these  deposits. 

The  deposits  show  certain  similarities  to  those  of  the  Sudbury 
district.  The  ore-minerals  are  the  same,  and  they  occur  at  the  base  of  a 
large  "basic"  sill.  They  differ  from  the  Sudbury  deposits  in  certain 
respects,  however,  and  features  that  are  hidden  at  Sudbury  are  exposed 
by  the  erosion  of  the  mountain  mass  of  the  Insizwa  range.  Du  Toit's 
results  are  summarized  in  the  following  paragraphs. 

Interest  has  been  aroused  in  these  deposits,  first,  by  the  discovery 
of  copper,  then  nickel,  and  recently  of  platinum.  The  ores  consist  of 
pyrrhotite,  pentlandite,  chalcopyrite,  with  some  pyrite,  niccolite,  and  a 
platinum  mineral,  probably  sperrylite.  The  ores  occur  at  the  base  of 
the  largest  of  a  series  of  sills  of  olivine  gabbro  and  olivine  norite.  The 
country  rock,  therefore,  is  more  "basic"  than  the  Sudbury  "nickel  in- 
trusive." At  this  locality  an  extensive  series  or  group  of  sills  occurs 
in  the  Beaufort  shales  and  sandstones  of  the  Karoo  system.  The  sills 
are  flat-topped,  but  the  under  surface  is  funnel-shaped,  narrowing  down 
into  the  feeding  dikes.  Strange  to  say,  the  intruded  shales  are  not  tilted 
but  are  "undisturbed  and  almost  absolutely  flat,  bounded  by  an  under 
regularly  curved  surface  of  fractures." 

The  ores  are  intimately  mixed  with  the  silicates  and  "intrude"  the 
country  rock  to  some  extent.  The  silicates  of  the  sill  show  no  altera- 
tion where  they  adjoin  the  ores.  In  addition  to  the  ores,  a  gran- 
itic extract  has  been  segregated  along  the  base  of  the  sill.  This 
occurs  as  a  network  of  dikes  and  dikelets,  from  a  fraction  of  an  inch 
to  a  foot  in  thickness.  Intense  contact  action  has  affected  the  sedimen- 
tary rocks,  and  seems  to  be  closely  related  to  the  dikes.  The  product 
of  the  contact  metamorphism  is  a  hornfels,  described  as  a  quartz-cor- 
dierite-feldspar  rock  with  abundant  biotite  and  enstatite  in  places.  In 
the  calcareous  layers  epidote,  zoisite,  diopside,  enstatite,  wollastonite,  and 
garnet  are  developed. 

Du  Toit's  figure64  shows  relations  similar  to  those  we  have  found  in 
the  magmatic  ores  from  other  localities,  and  which  we  interpret  as  in- 
dicating a  replacement  of  the  gangue  by  the  ore-minerals.  Du  Toit 

63  Du  Toit,  A.  L. — Report  on  the  copper-nickel  deposits  of  the  Insizwa, 
Mount  Ayliff,  East  Griqualand,  Cape  of  Good  Hope.  Dept.  Mines,  I5th  Ann. 
Kept,  of  the  Geol.  Comm.,  111-142  (1910).  64  Loc.  cit.,  fig.  4,  p.  139. 


46  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

makes  the  order  of  formation  of  the  sulfids  the  reverse  of  that  deter- 
mined by  Campbell  and  Knight  for  the  Sudbury  deposits.  An  examina- 
tion of  Du  Toit's  sketch  65  shows  the  same  marginal  relations  of  chal- 
copyrite  to  the  other  sulfids  as  found  in  Sudbury,  and  we  venture  to 
suggest  that  further  microscopic  study  of  the  ore  will  show  that  the 
order  of  the  formation  of  the  sulfids  is  the  same  at  Mount  Ayliff  as 
in  all  other  known  deposits  of  this  type. 

Summarizing,  we  find  the  following  suite  of  events  connected  with 
intrusion  and  ore  formation  at  Mount  Ayliff: 

(1)  The  intrusion  of  a  complex  family  of  "basic"  sills  connected  by 
cross-cutting  dikes. 

(2)  The  crystallization  of  the  silicate  minerals  of  the  gabbro  and 
norite. 

(3)  The  development  of  the  sulfid  ores  in  certain  localities  in  the 
basal  portion  of  the  largest  of  the  sills  by  replacement  of  the  silicates, 
but  unaccompanied  by  the  development  of  secondary  silicates. 

(4)  The  squeezing  out  of  a  granitic  extract,  forming  a  complex 
set  of  small  dikes  at  the  base  of  the  sill,  and  the  development  of  intense 
contact  metamorphism  due  to  gases  accompanying  the  "acid  extract." 
Ore  formation  also  occurred  in  this  stage,  as  ore-minerals  are  found 
in  the  pegmatite  and  granitic  dikes. 

NORWAY 
GEOLOGY 

In  Norway,  Nature  has  constructed  a  series  of  small-scale  models 
to  illustrate  the  type  of  nickel,  copper,  and  iron  sulfid  deposits  of  mag- 
matic  origin.  These  instructive  examples  have  been  described  in  detail 
by  Vogt,60  whose  classic  work  has  established  the  existence  and  char- 
acteristics of  this  type  of  ore  deposits.  In  as  much  as  a  part  of  the 
microscopic  data  admit  of  explanation  according  to  Vogt's  hypothesis 
of  an  intrusive  sulfid  magma,  we  discuss  in  some  detail  his  conclusions 
as  to  the  origin  of  these  ores. 

In  many  of  the  Norwegian  occurrences  neither  the  ore  nor  the 
norite  has  suffered  metamorphism,  and  the  geological  relations  are  sim- 
ple, so  that  it  seems  possible  to  select  the  phenomena  that  are  char- 
acteristic of  this  type  of  deposits.  It  is  essential  for  an  understanding 
of  the  problems  that  this  be  done,  for  in  many  of  the  deposits  of  nickel- 
iferous  pyrrhotite  elsewhere  the  data  are  either  wanting  or  not  easily 

«8  Loc.  cit.,  fig.  3,  p.  137.  88  Cited,  p.  5. 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS 


47 


deciphered.  At  Sudbury,  for  instance,  it  is  difficult  to  compile  the  story 
of  ore  formation  from  the  geologic  records,  overcrowded  with  the  de- 
tails of  the  eventful  igneous  and  metamorphic  history  of  which  ore 
deposition  is  a  mere  episode.  There,  inclusions  of  mafic  material  may 
be  explained  either  as  "basic"  segregations  or  as  inclusions  of  "the  older 
norite,"  as  best  suits  the  hypothesis 
favored  by  the  individual  investigator. 
Felsic  masses  and  dikes  are  variously 
explained  as  inclusions  of  an  older 
granite,  as  "acid"  segregations  of  the 
norite,  or  as  a  distinctly  younger  in- 
trusive granite. 

In  addition  to  the  little  altered 
types,  some  of  the  Norwegian  depos- 
its are  modified  by  subsequent  meta- 
morphism,  and  in  others  the  ores  are 
accompanied  by  metasomatic  borders 
of  garnet  (fig.  5).  Hydrothermal 
alteration  with  the  formation  of  py- 
rite  appears  to  be  developed  where 
pegmatitic  intrusions  are  numerous 
and  active  mineralizers  were  present 

(%.  3). 

These  gradation  types  are  fully  as 
instructive  as  the  normal  occurrences, 
and  support  our  contention  that  the 
typical  magmatic  deposits  are  one 
phase  of  high-temperature  deposition, 
and  changes  in  the  character  of 
the  mineralizers  result  in  gradations 
towards  the  other  types. 

In  Norway  there  are  about  fifty 
deposits  of  pyrrhotitic  copper-nickel 
ores  which  occur  exclusively  at  or 
near  the  margins  of  small  norite  or 
gabbro  stocks.  The  stocks  are  round  or  elliptical  in  outline,  and  some 
are  funnel-shaped  in  cross-section.  Figs.  2  and  3,  taken  from  Vogt,  illus- 
trate typical  occurrences.  The  individual  intrusives  are  small,  rarely 
exceeding  a  few  hundred  feet  in  diameter,  and  the  amount  of  ore  in  each 
stock  is  roughly  proportional  to  its  size. 


Fig.  2.  Mfeinkjar  mine,  Norway. 
(Distance  across  sketch  about  500 
ft.)  (After  Vogt,  Zeit.  f.  prakt. 
Geol.,  plate  VI,  fig.  3,  opposite  p. 
133,  Jahrgang  1893.) 


48 


A  STUDY  OF  THE   MAGMATIC  SULFID  ORES 


PETROGRAPHY  AND  MINERALOGY 

The  ore-bearing  rock  may  be  classed  broadly  as  norite  ( ±  quartz, 
olivine,  and  diallage),  and  in  a  few  localities  it  is  so  extensively  uralit- 
ized  as  to  be  classified  by  some  authorities  as  a  "gabbro-diorite."  Each 
individual  stock  has  undergone  extensive  differentiation,  especially  in 
the  vicinity  of  the  ore  bodies.  The  basic  segregations  are  mixtures  of 
bronzite,  hornblende,  olivine,  and  biotite.  They  occur  in  masses,  smaller 
inclusions,  and,  at  Romsaas,  as  a  peculiar  orbicular  norite.  The  felsic 


3OO       40O       SOQ  M 


Fig.  3.     Erteli  mine,  Ringerike,  Norway.     (After  Vogt, 
Zeit.  f.  prakt.  Geol.,  fig.  8,  plate  V,  Jahrgang  1893.) 

products  of  differentiation  are  as  prominent  as  the  mafic,  and  occur  as 
masses  of  granite,  streaks  and  dikes  of  pegmatitic  and  aplitic  material, 
and  small  marginal  veinlets  of  aplite  and  pegmatite.  A  similar  differen- 
tiation appears  to  be  a  characteristic  of  all  deposits  of  this  class.  The 
margins  of  the  stocks  often  show  severe  fracturing  and  brecciation,  and 
the  ore  may  cement  these  igneous  breccias,  as  at  Sudbury. 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS 


49 


A/crite 


Ore. 


According  to  our  ideas,  it  would  appear  that  (i)  the  "basic"  por- 
tions of  the  norite  solidified  by  the  sinking  of  crystals;  (2)  portions  of 
this  "basic"  material  were  caught  up  as  inclusions  in  the  normal  norite, 
especially  along  the  margins  of  the  stocks;  (3)  the  main  mass  of  the 
rock  crystallized  as  norite;  (4)  fel- 
sic dikes  and  masses  were  squeezed 
out  as  marginal  segregates  during 
a  period  of  peripheral  brecciation, 
which  facilitated  the  concentration 
and  escape  of  mineralizers  from 
the  deeper  portions  of  the  magma; 
(5)  ore  deposition,  at  a  late  stage, 
was  preceded,  accompanied,  and 
followed,  by  the  "acid"  secretions, 
as  the  ore  is  found  in  the  felsic 
dikes,  is  later  than  the  silicates  of 
the  same,  and  the  main  ore  masses 
are  cut  by  the  dikes. 

The  ores  occur  chiefly  at  the  margins  of  the  stocks,  and  to  a  minor 
amount  as  segregations  within  the  norite,  and  also  as  impregnations  in 
the  schists  and  gneisses  in  which  the  norite  masses  are  enclosed.  As  has 
been  mentioned,  the  ores  are  further  localized  where  marked  differen- 


Fig.  4.  Detailed  cross-section  of  ore- 
body  at  the  Meinkjar  mine,  Nor- 
way. (After  Vogt,  Zeit.  f.  prakt. 
Geol.,  fig.  29,  p.  136,  Jahrgang 
1893.) 


Fig.  5.  Pyrrhotite  gabbro  from  Erteli  mine, 
Norway.  Pyrrhotite  veinlet  cutting  feldspar 
and  diallage  with  rim  of  garnet,  (x  100.) 
(After  Vogt,  loc.  cit.,  p.  139.) 


Fig.  6.  Pyrrhotite  veinlets 
cutting  hornblende.  Flaad 
mine,  Evje,  Norway, 
(x  50.)  (After  Vogt,  loc. 
cit.,  p.  139.) 


tiation  has  taken  place.  At  the  Flaad  mine,  especially,  the  ores  are  closely 
related  to  felsic  differentiates. 

The  ores  show  the  same  group  of  minerals  (chiefly  magnetite, 
pyrrhotite,  pentlandite,  and  chalcopyrite),  and  the  same  relation  to  each 
other  and  to  the  schists,  as  we  have  found  in  the  deposits  studied  else- 
where. Pyrite  is  mentioned  by  Vogt  as  of  common  occurrence  in  these 


50  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

deposits.  He  describes  it  especially  in  the  Erteli  mine  at  Ringerike, 
and  the  Flaad  mine  at  Evje.  Ores  from  both  these  localities,  furnished 
through  the  courtesy  of  Professor  J.  F.  Kemp,  were  examined  by  us. 
The  pyrite  from  Ringerike  occurs  as  euhedral  and  irregular  grains, 
cutting  both  the  other  ore-minerals  and  the  silicates.  Its  position  in 
the  sequence  of  mineral  formation  could  not  be  definitely  determined 
from  the  material  available.  Pyrite,  in  the  ore  from  Evje,  however, 
clearly  cuts  garnet  and  also  all  the  ore-minerals,  including  chalcopyrite, 
as  groups  of  anastomising  sub-parallel  veinlets  (fig.  48).  These  veinlets 
widen  into  irregular  areas,  and  even  subhedral  crystals  are  often  devel- 
oped. This  makes  it  probable  that  the  euhedral  and  subhedral  pyrite 
crystals  of  fig.  48,  as  well  as  the  veinlets,  are  later  than  the  pyrrhotite. 
Chlorite  and  calcite  cut  the  sulfids  in  sharp  veinlets,  and  occur  along  the 
pyrite  veinlets.  They  are  certainly  later  than  all  the  sulfid  minerals 
except  pyrite,  and  may  be  later  than  it. 

As  already  stated,  pyrite  is  not  commonly  found  in  the  magmatic 
ores.  At  Sudbury,  it  is  found  especially  in  the  Worthington  offset, 
connected  with  later  veins  of  hydrothermal  origin.  In  the  Ringerike 
deposits  ore  has  been  mined  from  disseminations  in  the  country  rock, 
and  the  pyrite  in  these  ores  may  have  been  developed  in  the  schists 
prior  to  the  introduction  of  the  ores.  At  Evje,  as  stated  above,  the  pyrite 
is  the  last  sulfid  mineral  to  form,  and  may  have  been  developed  during 
the  extensive  uralitization  of  the  country  rock.  A  complete  microscopic 
study  of  all  the  Norwegian  ores  according  to  modern  petrographic  and 
metallographic  methods  is  needed  in  order  to  determine  the  paragenesis 
of  the  pyrite. 

As  described  and  figured  by  Vogt  (fig.  5),  veinlets  of  sulfid  ores 
are  accompanied  by  garnet,  showing  a  gradation  of  these  deposits  to 
other  high-temperature  deposits. 

Contrary  to  the  general  impression,  Vogt  states  that  the  ores  in  all 
cases  are  later  than  the  silicates.  He  notes  67  the  frequent  occurrence  of 
microscopic  veinlets  in  the  cracks  and  cleavage  planes  of  the  silicate 
minerals.  We  wish  to  emphasize  again,  that  these  data,  checked  by  us, 
admit  of  only  two  explanations,  viz.,  (i)  that  of  Vogt,  who  postulates 
an  intrusion  of  molten  sulfids  into  the  solidified  rock;  or  (2),  our  hy- 
pothesis that  the  sulfids  were  formed  by  the  replacement  of  silicates 
under  the  influence  of  mineralizers.  This  process,  however,  belongs  to 
a  late  magmatic  period,  and  not  to  destructive  pneumatolytic  or  hydro- 
thermal  stages,  and  the  deposits,  therefore,  may  properly  be  designated 
as  magmatic. 

«7Zeit.   f.  prakt.  Geol.,  Jahrgang  1893,  p.   139. 


I  DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  51 

DISCUSSION  OF  VOGT'S  CONCLUSIONS 
In  the  light  of  the  data  summarized  in  the  preceding  pages  we  may 
now  examine  in  detail  the  data  listed  by  Vogt,68  in  his  latest  contribu- 
tion, as  favoring  his  hypothesis. 

"As  suggested  more  particularly  by  Vogt  in  1893,  the  nickel-pyrrho- 
tite  deposits  are  to  be  regarded  as  magmatic  segregations  in  gabbro  or 
in  chemically  analogous  dike  rocks.  The  following  points  speak  for 
such  an  origin : 

1.  The  connection  of  the  numerous  deposits  in  different  countries 
with  occurrences  of  gabbro  or  exceptionally  with  equivalent  dike  rocks, 
is  constant  and  regular. 

2.  The  several  occurrences  so  resemble  one  another  not  only  geolog- 
ically but  also  mineralogically  that  they  must  be  of  the  same  genesis 
throughout,  while  the  unvarying  character  of  the  deposits  postulates  a 
simple  process  of  formation. 

3.  Gradations   between   ore   and   gabbro   through   the   intermediate 
stage  of  pyrrhotite-gabbro  are  often  to  be  observed,  and  therefore  the 
ore  essentially  must  have  been  formed  in  a  similar  manner  to  that  rock. 

4.  The  structure  of  the  clean  or  almost  clean  sulphide  mixture  with 
idiomorphic   crystals   suggests   crystallization    from   a   single   magmatic 
solution,  and  not  from  several  solutions  following  one  another,  as  was 
evidently  the  case  for  example  with  the  lead-silver-zinc  lodes. 

5.  The  deposits  often  occur  in  those  parts  of  a  gabbro  mass  which 
are  distinguished  by  pronounced  differentiation  of  the  eruptive  rock. 

6.  The    deposits    are    sometimes    crossed   by   dikes   of   basic    rock, 
diabase,  olivine-diabase,  etc.,  which  are  to  be  regarded  as  later  effusions 
of  the   same   eruption  of  gabbro.     The   formation  of  the  ore  belongs 
therefore  to  the  magmatic  period  of  the  rock  in  which  it  occurs. 

7.  Some  of   the   deposits   are  traversed   and  accompanied  by  acid 
leucocratic  streaks  and  dikes  which  represent  the  acid  segregation  prod- 
ucts from  the  gabbro-magma,  from  which  also  it  follows  that  the  for- 
mation of  the  deposits  took  place  during  the  magmatic  period  of  the 
eruptive  rock. 

8.  The  characteristic  presence  of  titanomagnetite  allows  a  manner 
of   formation  analogous  to  that  of  the  magmatic  titaniferous-iron   de- 
posits to  be  postulated. 

9.  Pneumatolytic  minerals  are  completely  wanting. 

10.  The  minerals  usually  formed  in  the  wet  way  are  not  present  as 
part  of  the  primary  formation." 

The  points  in  paragraphs  I  and  2  are  verified  by  our  work. 
The  ores  are  so  closely  connected  with  a  particular  type  of  basic  rock 

68  Ore  Deposits :  Beyschlag,  Vogt,  and  Krusch ;  Translated  by  S.  J.  Trus- 
cott;  pp.  286-287.  We  have  revised  the  translation  slightly  for  the  sake  of 
accuracy. 


52  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

that  a  genetic  relation  cannot  be  doubted.  The  majority  of  the  deposits 
are  so  similar  that  one  method  of  ore  formation  alone  is  involved.  The 
various  complicated  sets  of  minerals  produced  by  hydrothermal  solu- 
tions are  either  lacking  or  are  developed  after  the  formation  of  the  ores, 
and  hence  a  close  relation  to  rock  crystallization  is  suggested. 

Paragraph  3  has  little  meaning.  Variations  in  amount  of  re- 
placement, from  incomplete  to  complete,  are  found  in  most  ores  formed 
by  this  process.69  The  same  relations  obtain  in  magmatic  ores. 

In  regard  to  paragraph  4,  Vogt  is  in  error  as  to  the  facts.  Appar- 
ently he  had  not  studied  polished  ore  surfaces,  nor  has  he  grasped  the 
meaning  of  the  data  set  forth  in  the  articles  by  Dickson  70  and  by  Camp- 
bell and  Knight.71  The  ores  of  the  magmatic  stage  are  introduced  into 
the  completely  crystallized  rock,  one  after  the  other,  in  the  following 
order:  (i)  magnetite  and  ilmenite,  (2)  pyrrhotite,  (3)  pentlandite,  and 
(4)  chalcopyrite.  There  is  positive  evidence  of  a  certain  amount  of 
the  replacement  of  the  earlier  ore-minerals  by  those  introduced  at  later 
stages.  This  replacement  cannot  be  explained  by  corrosion,  for  there 
are  no  intermediate  reaction  products.  The  replaced  minerals  are  com- 
pletely removed  by  the  same  agencies  that  brought  in  the  ores.  Vogt 
recognizes  that  some  of  the  chalcopyrite  is  later  than  the  other  ore- 
minerals.  He  states  :72  "Die  haufig  beobachtete  Anreicherung  des  Kup- 
ferkieses,  namentlich  in  den  peripheren  Teilen  der  Nickel-Magnetkies- 
lagerstatten  muss  auf  eine  besondere  magmatische  Differentiation  inner- 
halb  der  Sulfidmagmen  beruhen." 

To  apply  his  ideas  to  the  facts  now  established,  he  would  have  to 
postulate  a  triple  or  quadruple  differentiation  within  the  sulfid  magma. 

The  points  in  paragraphs  5,  6,  and  7  are  verified  and  em- 
phasized by  us.  Magmatic  differentiation  is  characteristic  of  the  ore- 
bearing  norite,  and  is  especially  marked  in  the  vicinity  of  the  ores.  Ex- 
tensive early  differentiation  calls  for  an  equivalent  development  of  the 
complementary  later  differentiation.  The  ores  are  a  phase  of  the  latter. 
They  are  extracted  from  the  magma  basin  by  mineralizers,  and  are 
brought  to  the  margin  of  the  deposit  and  deposited  without  the  devel- 
opment of  secondary  silicates. 

Point  8  is  valid. 

«» Knight  (loc.  cit.)  emphasizes  this  in  his  recent  discussion  of  the  Sud- 
bury  ores. 

™  Loc.  cit.  71  Loc.  cit. 

«  Beyschlag,  Krusch,  Vogt.— Die  Lagerstatten  der  nutzbaren  Mineralien 
und  Gesteine.  Band  i,  284-285  (1910). 


DESCRIPTION  OF  THE  PYRRHOTITIC  DEPOSITS  53 

Point  9  is  true  in  general,  altho  tourmaline  and  garnet  are  occa- 
sionally present  and  hornblendization  generally  precedes  ore  deposition. 
Vogt  evidently  assumes  that  high-temperature  deposits  formed  by  min- 
eralizers  must  be  accompanied  by  destructive  pneumatolysis.  In  this 
regard  it  is  interesting  to  note  that  Vogt's  early  studies  led  him  to  sug- 
gest a  "combination  magmatic  segregation  with  pneumatolysis,  accord- 
ing to  which  metalliferous  vapors  evolved  from  the  magmas  in  one 
place  were  decomposed  in  another,  depositing  the  ore  in  rock  already 
consolidated."73  It  is  needless  to  state  that  his  early  concept  approached 
more  closely  to  the  truth  than  his  present  hypothesis. 

Paragraph  10  may  be  more  accurately  stated  as  follows:  The 
secondary  silicates  usually  formed  by  destructive  pneumatolysis  and  hy- 
drothermal  action  are  not  developed  in  the  magmatic  stage  of  ore  for- 
mation. 

SUMMARY 

We  conclude  with  Vogt  that  the  ores  are  so  closely  related  to  the 
intrusive  rocks  which  contain  them,  and  to  the  processes  of  rock  differ- 
entiation, and  differ  so  markedly  from  deposits  formed  by  pneumatolytic 
and  hydrothermal  processes,  that  they  should  be  classed  as  magmatic. 
However,  they  cannot  be  considered  as  segregated  in  the  molten  stage  by 
a  "liquation"  process,  and  at  a  later  date  intruded  into  the  silicates,  for 
the  following  reasons : 

1.  The  ore-minerals  are  formed  in  an  orderly  sequence,  one  after 
the  other.     A  succession  of  sulfid  differentiations  and  intrusions  is  be- 
yond the  realm  of  probability. 

2.  The  ores  replace  the   country  rock  without  reaction  rims,  and 
the  silicates  thus  replaced  have  been  completely  removed. 

3.  The   minerals   are  introduced  after  the   magmatic  alteration   of 
pyroxene  to  hornblende,  but  probably  prior  to  the  intense  uralitization 
certain  of  the  deposits  have  undergone. 

4.  The  temperature  of  the  late  stages  of  differentiation  in  the  pres- 
ence of  mineralizers,  and  the  formation  of  pegmatite  and  aplite  dikes, 
is  far  below  the  temperature  of  molten  ores,  as  we  know  them  as  fur- 
nace products. 

5.  The  phenomena  of  ore  formation  and  rock  replacement  are  sim- 
ilar in  all  respects  to  that  of  sulfid  ore  deposition  from  hydrothermal 
solutions,  except  for  the  comparative  absence  of  secondary  silica  and 
silicates.    This  is  a  characteristic  feature  of  this  type  of  ore,  and  is  due 
to  the  conditions  of  chemical  equilibria  under  which  they  are  formed. 


"Truscott  trans.     Loc.  cit,  289. 


54  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 


BIBLIOGRAPHY  OF  THE  NORWEGIAN  DEPOSITS 

Vogt,  J.  H.  L. — Bildung  von  Erzlagerstatten  durch  Differentiationsprocesse  in 
basichen  Eruptivmagmata.  Zeit.  f.  prakt.  Geol.,  Jahrgang  1893,  pp.  4-11,  125- 
143,  257-284. 

See  also  Stelzner-Bergeat,  Die  Erzlagerstatten,  I,  48,  1904,  for  literature  of  the 
Norwegian  occurrences. 


OTHER  EUROPEAN  OCCURRENCES 

SWEDEN 

The  nickel-  and  copper-bearing  pyrrhotite  deposits  of  Sweden  are 
similar  to  the  more  extensive  group  in  Norway.  They  appear,  in  gen- 
eral, to  have  suffered  greater  alteration  than  the  latter.  The  largest 
deposit  is  at  Klefva,  in  Smaland.  The  ores  occur  in  a  quartz  norite 
which  has  suffered  intense  uralitization.  Some  deposits  at  Kuso  and 
Stattberg  occur  in  mafic  (diabase)  dikes,  and  are  somewhat  similar  to 
the  occurrence  at  Sohland  mentioned  below.74 

BADEN,  HORBACH,  AND  TODTMOOS,  GERMANY 

In  the  southern  Black  Forest  pyrrhotitic  ores  occur  in  small  amount 
in  basic  inclusions  and  dikes,  extensively  altered  to  amphibolite  and  ser- 
pentine, which  occur  in  granite  and  gneiss.  They  are  cut  by  granite- 
aplite  dikes.75 

SOHLAND  AND  SWEIDERICH  ON  THE  BOUNDARY  BETWEEN  SAXONY  AND 

BOHEMIA 

Nickel-  and  copper-bearing  pyrrhotite76  impregnates  the  footwall  of 
irregular  "basic"  dikes  which  are  varied  in  composition  on  account  of 
pronounced  differentiation. 

The  ores,  according  to  Beck,  surround  and  cut  corroded  augite, 
hornblende,  and  biotite,  and  shatter  and  impregnate  the  latter.  Beck  was 

74  Literature  is  cited,  Beyschlag,  Krusch,  Vogt,  I,  p.  294,  and  Stelzner-Ber- 
geat, I,  p.  48. 

75  Weinschenk,   E.— Die   Nickelmagnetkieslagerstatten   in   Bezirk   St.   Blasien. 
Zeit.   f.  prakt.   Geol.,  Jahrg.   1907,  pp.   73-86.     Other  literature   cited  in  the  text- 
books of  Beck  and  Stelzner-Bergeat. 

76  Beck.    R. — Die    Nickelerzlaperstatten    von    Sohland    a.    d.    Spree    und    ihre 
gesteine.    Zeit.  deutsch.  geol.  Ges.,  Jahrg.  1903,  pp.  296-331.     (Includes  bibliography 
of  the  older  literature.) 

Lehre  von  Erzlagerstatten,  I,  81-86  (1909). 

von  Foullon,  H.  B.  Uber  einige  Nickelvorkommen.  Jahrb.  k.k.  geol. 
Reichsanst.  302  pp.  Wein.  (1892). 


DESCRIPTION   OF  THE  PYRRHOTITIC  DEPOSITS  55 

unable  to  distinguish  pentlandite  from  pyrrhotite,  but  we  found  no  dif- 
ficulty in  showing  its  existence  in  polished  surfaces  (see  figs.  50  and  51, 
plate  XII). 

Beck  has  proved  conclusively  that  the  ores  are  later  than  the  pri- 
mary silicates,  and  believes  that  they  are  formed  by  a  "post-volcanic, 
pneumatolytic  phase  of  rock  building."  77 

We  have  a  small  suite  of  specimens  from  Sohland  which  include 
both  the  unaltered  rock  and  ores.  These  we  have  examined  in  thin  sec- 
tions, and  the  ores  also  in  polished  sections.  The  unaltered  rock  is  a 
hornblende-diabase  (called  proterobase  by  Beck),  containing  plagioclase, 
augite,  brown  hornblende,  biotite,  apatite,  titanite,  and  magnetite.  The 
only  alteration  products  present  are  a  little  calcite,  tremolite,  and  chlorite. 
The  hornblende  occurs  in  parallel  position  with  the  augite  and  is  evi- 
dently a  late  magmatic  mineral. 

In  thin  sections  of  the  ore  the  only  original  silicate  mineral  noted 
is  the  hornblende.  The  feldspars  and  biotite  are  altered  almost  beyond 
recognition.  The  alteration  products  are  chlorite,  sericite,  and  uralite. 
Of  especial  interest  is  the  uralite,  because  its  relation  to  the  ore-minerals 
can  be  definitely  established.770 

Fig.  50  shows  the  general  relations  of  the  minerals  in  the  polished 
sections.  A  study  of  our  sections  together  with  Beck's  original  figures 
(especially  his  fig.  4  of  plate  XIII)  makes  it  almost  certain  that  the  ore- 
minerals  were  formed  later  than  the  hornblende,  but  earlier  than  the 
uralite.  Uralite  develops  on  the  ends  of  the  hornblende  prisms  in  par- 
allel position,  and  occasionally  completely  replaces  the  hornblende.  Fre- 
quently the  uralite  needles  project  out  into  the  ore-minerals,  as  in  fig.  51, 
and  in  Beck's  fig.  4.  This  affords  conclusive  evidence  of  post-mineral 
alteration.  For  this  reason  we  believe  that  the  Sohland  ore  is  magmatic 
in  spite  of  the  extensive  alteration.  The  relation  of  the  ore-minerals  to 
the  silicates  was  evidently  established  during  the  magmatic  period,  and 
not  modified  by  later  hydrothermal  action. 


77  Quoted  by   Berg,   opus   cit,    108. 

77°  In  the  literature  the  term  uralite  has  been  used  in  two  senses,  (i)  for 
hornblende  rims  around  pyroxene,  (2)  for  the  fine  fibrous  aggregates  of  tremolite 
(including  actinolite).  The  first  usage  we  have  avoided  entirely,  and  in  the  few 
cases  in  which  the  terms  uralite  and  uralitization  are  used  by  us,  these  are  to  be 
given  the  second  meaning.  The  hornblende  rims  appear  to  be  late  magmatic  and 
the  tremolite  a  post-magmatic,  hydrothermal  mineral. 


56  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 


GROUP  II.  MAGMATIC  CHALCOPYRITE-BORNITE  DEPOSITS 


OOKIEP,  NAMAQUALAND,  SOUTH  AFRICA 
GEOLOGY 

The  important  mines  of  this  region  occur  as  great  lenticular  deposits 
in  the  vicinity  of  the  town  of  Ookiep,  Namaqualand,  Cape  Colony,  South 
Africa.  They  lie  about  sixty  miles  east  from  the  Atlantic  coast,  and 
about  twice  as  far  south  of  the  Orange  river. 

The  ore  bodies  occur  as  shoots  or  lenses,  and  again  as  disseminated 
sulfid  particles,  in  a  system  of  dikes  and  sills,  which  give  evidence 
of  remarkably  extensive  differentiation.  Rogers78  describes  the  "ore 
bearer"  as  including  rock  varieties  made  up  almost  wholly  of  any  one 
of  the  following  minerals:  magnetite,  quartz,  plagioclase,  hypersthene, 
hornblende,  and  biotite,  and  also  intermediate  varieties  such  as  norite, 
mica  diorite,  augite  diorite,  and  diorite.  A  single  dike  may  show  one  or 
more  varieties  of  rock,  and  where  several  kinds  occur  in  one  intrusive 
the  contacts  are  sharp,  suggesting  differentiation  before  intrusion. 

According  to  Kuntz  79  the  intrusive  rock  occurs  as  dikes  of  great 
extent,  or  again  as  detached  stocks.  To  these  Rogers  adds :  pipes,  nearly 
horizontal  sheets,  and  branching  bodies ;  and  states  that  344  intrusive 
bodies  have  been  mapped  up  to  date.  These  dikes  cut  the  "fundamental 
gneiss"  of  South  Africa,  and  the  ore  occurs  as  shoots,  occupying  a  por- 
tion of  the  entire  width  of  the  dikes,  and  also  penetrates  the  gneiss  ad- 
jacent to  the  dikes. 

The  principal  ore-minerals  are  given  as  magnetite,  pyrrhotite,  born- 
he,  and  chalcopyrite.  We  find  that  hematite  is  also  an  important  con- 
stituent of  the  ores. 

The  chief  mines  are  Ookiep,  Specktakel,  Nababeep,  and  Ookiep 
East,  all  situated  near  the  town  of  Ookiep,  and  the  Tweefontein  mine  is 
located  on  a  dike  to  the  north.  The  ore  shoots  are  lenticular  masses  of 
high  grade  ore  with  greatest  dimension  in  a  horizontal  direction.  For 
example,  the  Ookiep  ore-body,  according  to  Ronaldson,80  is  1300  feet 

78  Rogers,  A.  W. — The  nature  of  the  copper  deposits  of  Little  Namaqualand, 
Proceed.  Geol.   Soc.   S.  Africa,   1916,  pp.  xxi-xxxix. 

79  Kuntz,   J. — Copper    ore    in    southwest    Africa.      Trans.    Geol.    Soc.    South 
Africa,  7,  70-76  (1904). 

80  Ronaldson,  J.  H. — Notes  on  the  copper  deposits  of  Little   Namaqualand. 
Trans.  Geol.  Soc.  S.  Africa,  8,   158-167   (1905). 


CHALCOPYRITE-BORNITE  DEPOSITS  57 

wide  and  300  feet  deep.     The  Tweefontein  mine  contains  a  series  of 
three  lenses,  one  under  the  other. 

On  account  of  the  occurrence  of  the  copper  ore  intimately  mixed 
with  apparently  unaltered  constituents  of  the  dikes,  a  magmatic  origin 
was  early  assigned  to  these  deposits.  In  1857,  Wiley81  stated  that  the 
ores  were  of  magmatic  origin,  and  Daintre  in  1878  makes  the  plain 
statement  that  "the  ores  are  of  magmatic  origin,  occurring  in  unaltered 
feldspathic  dikes  from  specks  the  size  of  a  pin  point  to  many  tons."  82 

MICROSCOPIC  DESCRIPTIONS 

We  are  indebted  to  Dr.  A.  W.  Rogers,  Director  of  the  Geological 
Survey  of  the  Union  of  South  Africa,  for  an  excellent  suite  of  speci- 
mens of  ores  and  accompanying  rocks  from  the  Ookiep  mines.  These 
rocks  vary  from  an  almost  pure  hypersthene  rock  (hypersthenite), 
through  norite  and  mica  diorite,  to  an  almost  pure  plagioclase  rock 
(anorthosite).  The  specimens  include  both  lean  and  rich  norite  and 
mica  diorite,  but  the  anorthosite  is  almost  free  from  the  ore-minerals, 
and  the  hypersthenite  carries  a  fair  amount  of  the  ore-minerals. 

Ore-bearing  Hypersthenite  from  the  Nababeep  Mine. — The  general 
relations  of  the  ore-minerals  to  the  silicates  of  the  hypersthenite  are  well 
shown  in  fig.  54.  The  ore-minerals  occur  in  anhedra  which  are  often 
elongate  and  pass  into  vein-like  forms.  It  is  probable  that  bornite  has 
formed  by  the  replacement  of  the  magnetite,  for  they  show  the  same 
general  shape. 

Fig.  52  represents  a  thin  section  of  the  hypersthenite.  The  trans- 
parent mineral  is  hypersthene.  The  black  area  is  mainly  magnetite,  but 
a  little  bornite  is  also  present,  and  in  some  places  in  the  section  bornite 
predominates  over  magnetite.  The  ore-minerals  surround  the  hyper- 
sthene crystals,  and  in  one  place  (just  below  the  center)  a  veinlet  of  mag- 
netite cuts  across  a  hypersthene  crystal. 

Besides  hypersthene  the  only  original  silicates  present  are  biotite 
and  plagioclase,  which  occur  in  minor  amounts.  The  plagioclase  is 
shown  in  little  patches  in  fig.  53.  The  association  of  ores  with  these 
areas  of  feldspar,  which  considered  with  respect  to  the  enclosing  rock 
are  of  the  nature  of  "acid  extracts,"  is  significant.  The  feldspar  seems 
to  be  more  readily  replaced  by  the  ore-minerals  than  the  hypersthene. 
Note  the  small  magnetite  euhedra  within  the  hypersthene. 

81  Wiley,    A. — Report    on    the    mineral    and    geological    structure    of    South 
Namaqualand.     Parliamentary  Report,  Cape  Town,  no.  36,  p.  30   (1857). 

82  Quar.  Jour.  Geol.  Soc.,  34,  434   (1878). 


58  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

The  minerals  of  the  hypersthenite  are  comparatively  fresh,  but 
there  has  been  some  alteration  along  the  boundaries  between  the  hyper- 
sthene  anhedra.  This  alteration  product  of  hypersthene  is  prob- 
ably anthophyllite.  It  occurs  in  fibrous  aggregates  which  are  often 
intimately  associated  with  the  bornite.  Fig.  55  is  a  photomicrograph  of 
an  area  showing  the  relations  of  the  anthophyllite  to  the  bornite.  A 
careful  study  of  the  section  shows  that  the  anthophyllite  has  replaced 
the  bornite.  The  narrow  linear  areas  of  bornite  might  be  considered 
evidence  that  the  bornite  had  replaced  anthophyllite ;  but  that  the  reverse 
is  true  is  proved  by  the  obliquely-cutting  anthophyllite  needles  shown  at 
several  points. 

Ore-bearing  Norite  from  the  Tweefontein  Mine. — Sections  of  ore- 
bearing  norite  from  the  Tweefontein  mine  are  represented  on  plates 
XIV  and  XV.  The  silicate  minerals  are  hypersthene  and  plagioclase, 
with  subordinate  biotite.  The  ore-minerals  are  magnetite,  chalcopyrite, 
and  bornite.  Fig.  56  illustrates  the  replacement  of  hypersthene  by  ore- 
minerals,  principally  magnetite  with  a  little  bornite  and  chalcopyrite.  At 
the  bottom  of  the  photograph  there  is  a  small  magnetite  crystal,  which 
is  anhedral  along  the  border  between  the  hypersthene  and  plagioclase 
and  euhedral  within  the  plagioclase  crystal.  This  is  better  shown  in 

fig-  57- 

In  fig.  58  the  opaque  mineral  is  largely  bornite,  with  a  little  chalcopy- 
rite and  magnetite.  At  the  bottom  of  the  figure  a  hypersthene  crystal 
is  almost  completely  surrounded  by  bornite.  A  higher  magnification  of 
the  upper  left  corner  of  this  area  is  shown  in  fig.  59.  The  bornite  has 
replaced  the  plagioclase  in  the  direction  of  twinning  lamellae. 

Figs.  60-63  (plate  XV)  show  the  occurrence,  in  the  Tweefontein 
norite,  of  the  ore-minerals  in  polished  sections.  The  general  relations 
are  shown  in  fig.  60.  Magnetite  and  hematite  are  readily  distinguished 
by  the  character  of  their  surfaces  (fig.  61).  Magnetite  is  rough  and 
is  intergrown  with  ilmenite;  hematite  is  smooth.  Magnetite  and  hema- 
tite, as  well  as  the  sulfids,  surround  and  replace  the  silicates.  The  re- 
placement of  biotite  by  the  sulfids  is  well  shown  in  fig.  60.  Note  the 
veinlike  magnetite  which  cuts  directly  through  a  hypersthene  crystal. 

The  norite  specimens  from  the  Tweefontein  mine  are  remarkably 
free  from  alteration,  as  can  be  seen  from  the  photomicrographs.  There 
was,  however,  a  little  alteration  along  minor  fractures  (fig.  62)  in  born- 
ite. Along  one  of  these  fractures  (illustrated  by  fig.  63)  gashes  of  chal- 
copyrite of  a  second  generation  and  minute  crystals  of  anthophyllite 
were  developed.  The  high  magnification  of  these  figures  shows  how 
insignificant  is  the  alteration. 


CHALCOPYRITE-BORNITE  DEPOSITS  59 

Another  specimen  of  norite  taken  from  drill-cores  at  the  Tweefon- 
tein  mine  constitutes  a  lean  ore.  Very  small  amounts  of  bornite  and 
chalcopyrite  are  visible.  There  is,  however,  considerable  magnetite.  A 
study  of  the  thin  section  shows  as  good  evidence  of  the  replacement  of 
the  silicates  as  one  could  wish  for.  A  small  apatite  crystal  has  been  cut 
squarely  in  two  by  magnetite. 

Ore-bearing  Mica  Diorite  from  the  Ookiep  East  Mine. — There  re- 
mains to  be  described  the  mica  diorite  from  the  Ookiep  East  mine.  The 
principal  minerals  are  plagioclase  and  biotite.  The  alteration  products 
include  anthophyllite,  clinozoisite,  and  chlorite.  The  ore-minerals  are 
magnetite,  pyrrhotite,  and  chalcopyrite;  bornite  is  absent.  The  mica 
diorite  is  poor  in  ore,  and,  strange  to  say,  is  more  altered  than  that  con- 
taining large  amounts  of  chalcopyrite  and  pyrrhotite.  Fig.  65  illustrates 
the  occurrence  of  the  ore-minerals  in  the  rich  ore  as  seen  in  thin  section. 
The  alteration  veinlet  (chlorite)  at  the  bottom  of  the  figure  is  later  than 
the  sulfids.  The  light  spots  within  the  opaque  areas  are  also  alteration 
products. 

A  study  of  the  polished  sections  with  a  low-power  microscope  indi- 
cates that  the  chalcopyrite  is  probably  formed  by  the  replacement  of 
the  pyrrhotite  (see  fig.  64).  Another  spot  furnishes  evidence  that  the 
pyrrhotite  has  in  part  been  formed  by  the  replacement  of  magnetite  (see 

%.  67). 

Some  portions  of  the  specimens  of  rich  ore  from  the  Ookiep  East 
mine  contain  definite  sulfid  veinlets.  These  are  shown  in  thin  section 
in  fig.  66  and  in  polished  section  in  fig.  68.  These  veinlets,  unlike  the 
sulfid  veinlets  in  the  Sudbury  ores,  have  a  peculiar  "fuzzy"  appearance 
which  (see  fig.  66)  suggests  that  they  might  have  been  produced  by  re- 
arrangement of  the  larger  sulfid  masses.  The  study  of  the  thin  sections 
under  high  magnification  shows  that  this  appearance  is  due  to  the  pres- 
ence of  a  secondary  silicate  (chlorite  or  anthophyllite).  That  the  vein- 
lets  are  not  due  to  rearrangement  is  proved  by  fig.  69,  which  distinctly 
shows  that  the  alteration  product,  probably  chlorite  formed  by  the  alter- 
ation of  anthophyllite,  is  later  than  the  chalcopyrite  of  the  veinlets. 
Minute  specks  of  a  light  yellow  mineral,  probably  pentlandite  (fig.  71), 
were  also  found  in  the  ore  from  the  Ookiep  East  mine.  It  is  a  late, 
probably  hydrothermal,  mineral,  and  furnishes  the  only  evidence  of  re- 
arrangement of  the  sulfids  in  our  specimens  of  the  ores  of  the  Ookiep 
East  mine. 

The  replacement  of  the  silicates  is  shown  in  all  the  photographs  in 
plates  XVI  and  XVII,  but  especially  well  in  fig.  70.  This  represents  a 


60  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

biotite-rich  segregation  in  the  diorite.  The  biotite,  itself  probably  a  late 
magmatic  mineral,  has  been  replaced  along  its  cleavage  planes  by  chal- 
copyrite.  This  produces  a  structure  which  can  be  distinguished  from  that 
developed  by  the  replacement  of  the  sulfids  by  later  hydrothermal  min- 
erals such  as  sericite,  chlorite,  and  tremolite.  (See  for  examples  figs. 
82  and  83,  plate  XX.) 

SUMMARY 

The  Ookiep  ores  are  of  importance  in  establishing  the  type  of  mag- 
matic copper  deposits.  All  who  have  studied  these  ores  have  classified 
them  as  magmatic  ores,  but  in  spite  of  this  they  have  received  scant  at- 
tention by  the  authors  of  textbooks  and  treatises  on  ore  deposits.  The 
principal  reason  for  the  failure  to  give  these  deposits  full  standing  in 
the  magmatic  class  is  the  presence  of  bornite.  The  Ookiep  ores  are 
magmatic,  if  such  exist.  They  are  the  least  altered  of  any  we  have 
examined  in  this  study.  The  ore-bearing  (chalcopyrite-bornite)  norite 
from  the  Tweefontein  mine  is  much  freer  from  alteration  than  the  aver- 
age igneous  rock.  It  is  true  that  the  mica  diorite  and  hypersthenite  at 
Ookiep  locally  show  some  alteration,  but  the  significant  thing  is  that  the 
ore-minerals  in  the  altered  and  unaltered  rocks  show  exactly  the  same 
relation  to  the  silicate  minerals. 

BIBLIOGRAPHY  OF  THE  OOKIEP  DEPOSITS 

Delesse,   M. — Sur  les  mines   de  cuivre   du   Cap   de   Bonne   Esperance.     Ann.   des 

Mines,  5me  serie,  8,  186-212  (1855). 
Wiley,  A. — Report  on  the  mineral  and  geological  structure  of  South  Namaqualand. 

Parliamentary  Report,  Cape  Town,  no.  36,  p.  30  (1857). 
Zerrener,    C. — Reise    des    Ingenieurs    A.    Thiers    nach    den    Kupfer    Bergwerken 

Namaqualands  in  Sud  Africa.     Berg,  und  Hiittenm.     Zeitung  1860,  pp.  41-44 

and  53-54- 
Knopf,  A. — Uber  die  Kupfererzlagerstatten  von  Klein  Namaqualand  und  Damara- 

land.     Neues  Jahrb.  f.  Min.  Geol.  u.  Pal.,  1861,  pp.  513-550. 
Schenk,  A. — Die  Kupfererzlagerstatten  von  Ookiep  in  Klein  Namaqualand.     Zeit. 

de  deutsch.  geol.  Ges.  Verhand.  der  Ges.,  53,  64-65   (1902). 
Kuntz,  J. — Copper  ore  in  Southwest  Africa.     Trans.  Geol.   Soc.  South  Africa,  7, 

70-76  (1904). 
Kupfererzvorkommen    in    Siidwestafrika.      Zeit.    f.    prakt.    Geol.,    12,    199-202 

(1904). 
Ronaldson,  J.  H. — Notes  on  the  copper  deposits  of  Little  Namaqualand.     Trans. 

Geol.  Soc.  South  Africa,  8,  158-167  (1905). 
Stutzer,  O .— Magmatische  Ausscheidungen  von  Bornit.    Zeit.  f.  prakt.  Geol.,  15,  371 

(1907). 

Rogers,  A.  W—  The  nature  of  the  copper  deposits  of  Little  Namaqualand.     Pro- 
ceed. Geol.  Soc.  S.  Africa,  1916,  pp.  xxi-xxxiv. 


CHALCOPYRITE-BORNITE  DEPOSITS  61 

ENGELS  MINE,   PLUMAS  COUNTY,  CALIFORNIA 

GEOLOGY 

A  rather  unique  copper  deposit,  which  we  believe  to  be  magmatic, 
is  now  being  mined  by  the  Engels  Copper  Mining  Company  in  the 
northern  part  of  Plumas  county,  California.  This  deposit  has  been  studied 
and  described  by  Turner  and  Rogers.83 

The  ore  occurs  in  a  remarkably  fresh  norite-diorite  occurring  at 
the  extreme  northern  end  of  the  great  Sierra  Nevada  batholith  of  grano- 
diorite.  There  is  no  evidence  of  dynamic  metamorphism  and  none  of 
contact  metamorphism,  for  the  older  rocks  into  which  the  norite-diorite 
is  intrusive  are  nearly  five  miles  distant.  The  main  ore  body  is  a  tabu- 
lar ore  shoot  which  has  a  nearly  vertical  attitude.  It  is  difficult  to  see 
how  this  ore  body  could  possibly  be  explained  by  gravitative  adjustment 
due  to  the  sinking  of  sulfids  in  the  molten  magma.  Narrow  pegmatite 
dikes  are  occasionally  found  cutting  the  norite-diorite. 

MICROSCOPIC  DESCRIPTIONS 

The  predominant  igneous  rock  at  the  Engels  mine  is  a  norite-diorite 
with  about  46  per  cent  of  silica.  The  principal  minerals  are  plagioclase 
(andesine-labradorite,  Abj  A^  ),  hypersthene,  diopside,  hornblende,  and 
biotite.  The  norite-diorite  varies  from  a  rather  fine-grained  hypersthene- 
plagioclase  rock  to  a  coarse-grained  hornblende-plagioclase  rock.  One 
of  the  typical  fine-grained  rocks  (from  no.  2  level)  is  illustrated  in  fig. 
72.  The  opaque  minerals  here  are  magnetite  and  hematite.  They  sur- 
round and  replace  the  silicates,  especially  hypersthene  and  biotite.  Evi- 
dence of  replacement  is  shown  at  many  spots  in  the  photomicrograph. 
A  magnified  view  of  one  of  these  is  shown  in  fig.  74.  The  magnetite 
and  hematite  have  surrounded  hypersthene  and  plagioclase,  and  have 
penetrated  and  replaced  a  biotite  crystal.  It  will  be  noticed  in  fig.  72 
that  the  magnetite  occurs  in  both  euhedral  and  anhedral  crystals.  The 
euhedral  crystals  occur  within  the  silicate  minerals,  and  the  anhedral 
crystals  mainly  along  the  boundaries  of  adjacent  silicate  anhedra.  This 
is  also  well  shown  in  fig.  73.  In  this  figure  there  is  a  magnetite  crystal 
which  shows  anhedral  development  on  the  boundary  between  the  hy- 
persthene and  plagioclase,  and  euhedral  development  within  the  hyper- 
sthene. This  is  an  argument  in  favor  of  the  late  magmatic  origin  of 


83  A  geologic  and  microscopic  study  of  a  magmatic  copper  sulphide  deposit 
in  Plumas  county,  California,  and  its  modification  by  ascending  secondary  en- 
richment. Econ.  Geol.,  9,  359-3QI  (1914)- 


62  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

euhedral  magnetite,  as  is  also  the  fact  that  all  gradations  between  an- 
hedral  and  euhedral  forms  occur.  This  practically  proves  that  the  euhe- 
dral, as  well  as  the  anhedral,  magnetite  is  formed  by  the  replacement  of 
the  silicates  at  a  late  stage.  This  rock  is  practically  free  from  alteration. 

The  coarse-grained  norite-diorite  usually  contains  hornblende  in- 
stead of  hypersthene.  The  hornblende  in  these  rocks  has  probably  been 
formed  from  pyroxene.  Evidence  of  this  late  magmatic  alteration  is 
furnished  in  fig.  75,  where  residual  cores  of  diopside  and  hypersthene 
occur  within  a  hornblende  crystal. 

In  plate  XIX  are  shown  photomicrographs  of  a  specimen  somewhat 
similar  to  the  rock  just  described,  except  that  it  contains  alteration  pro- 
ducts. The  ferro-magnesian  minerals  are  hypersthene,  diopside,  and 
hornblende.  The  latter  occurs  as  rims  around  the  diopside  (fig.  76). 

The  ore-minerals  in  this  specimen  are  magnetite,  hematite,  chal- 
copyrite,  and  bornite.  The  magnetite  contains  regularly  arranged  ilmen- 
ite  plates.  The  hypersthene  is  extensively  altered  to  a  mineral 
with  indefinite  aggregate  structure.  This  contains  tremolite  and  proba- 
bly talc,  and  it  is  possible  that  tremolite  is  an  intermediate  product  of 
the  alteration  of  hypersthene  to  talc.  Chlorite,  another  alteration  prod- 
uct, occurs  in  veinlets  which  definitely  cut  the  ore-minerals  as  shown 
in  fig.  77. 

Covellite  and  chalcopyrite  of  the  second  generation  have  been  formed 
in  occasional  spots  at  the  expense  of  the  bornite  (fig.  81).  These  are 
the  result  of  the  rearrangement  brought  about  subsequent  to  the  mag- 
matic stage. 

Some  of  the  ore  at  the  Engels  mine  occurs  in  a  rather  fine-grained 
felsic  rock  containing  plagioclase,  orthoclase  (or  microcline),  quartz, 
and  biotite,  with  minor  accessories.  For  convenience  this  rock  is  called 
grano-diorite.  In  one  specimen  of  this  type  apatite,  titanite,  epidote, 
calcite,  and  analcite  were  found.  These  are  not  alteration  products  of 
any  minerals  present  in  the  rock,  and  are  considered  to  be  of  late  mag- 
matic origin  and  formed  by  mineralizers.  This  rock  is  remarkably  fresh ; 
the  only  hydrothermal  products  present  are  a  little  chlorite  formed  from 
biotite  and  a  little  sericite  from  feldspar. 

Another  type  of  the  fine-grained  grano-diorite  is  exceptional  in  that 
it  contains  tourmaline.  This  specimen,  which  is  figured  in  plate  XIX 
(figs.  78  and  79),  contains  a  good  deal  of  bornite  and  chalcocite.  The 
bornite  occurs  in  irregular  anhedra  which  surround  and  replace  the 
silicates.  The  tourmaline  has  also  been  replaced  by  bornite  (fig.  78). 
This  specimen  contains  considerable  chlorite,  which  is  evidently  pseudo- 
morphous  after  biotite. 


CHALCOPYRITE-BORNITE  DEPOSITS  63 

The  igneous  rocks  which  constitute  the  ores  at  the  Engels  mine 
contain  varying  amounts  of  alteration  products,  which  are  largely  seri- 
cite,  chlorite,  tremolite,  and  talc.  Some  specimens,  such  as  those  repre- 
sented by  fig.  72,  are  practically  free  from  alteration,  while  others  are 
considerably  altered.  The  hypersthene  is  frequently  altered  to  a  gray 
indefinite  substance  with  aggregate  structure  (fig.  77).  This  is  prob- 
ably talc.  It  is  probable  that  tremolite  is  an  alteration  product,  inter- 
mediate in  point  of  time  between  hypersthene  and  talc. 

Sericite  is  present  in  certain  specimens  in  fair  amounts.  It  occurs 
as  a  replacement  of  feldspar,  and  also  as  a  replacement  of  the  sulfids. 
This  is  shown  in  fig.  78.  The  sharp  lath-shaped  crystals  show  well  in 
contrast  with  the  black  bornite  (and  chalcocite),  but  exactly  the  same 
kind  of  crystals  appears  in  the  feldspars. 

The  chlorite  occurs  in  definite  veinlets  cutting  the  ore-minerals  (fig. 
77),  and  also  in  lath-shaped  sections  resembling  sericite  and  these  also  cut 
the  ore-minerals  (fig.  82). 

The  rearrangement  of  the  ore-minerals  in  magmatic  sulfid  deposits 
is  insisted  upon  by  several  writers  to  explain  certain  features.  In  the 
Engels  ore  the  rearrangements  brought  about  by  lowering  the  temper- 
ature are  well  illustrated.  The  magnetite  and  hematite  suffer  no  changes, 
but  the  bornite  is  often  broken  down  into  chalcocite,  covellite,  and  chal- 
copyrite  of  the  second  generation.  Examples  of  these  are  shown  in 
plate  XX.  The  so-called  graphic  intergrowth  of  bornite  and  chalcocite 
(fig.  80),  as  one  of  us  suggests  in  a  recent  paper,84  is  a  local,  very 
irregular,  replacement  of  bornite  by  chalcocite.  The  replacement 
of  bornite  by  chalcocite  also  takes  place  in  veinlets  (fig.  83),  and  occa- 
sionally along  crystallographic  directions ;  but  the  best  example  of  crys- 
tallographic  influence  during  replacement  is  the  break-down  of  the 
bornite  to  chalcopyrite  of  the  second  generation  (fig.  81).  Covellite 
also  replaces  bornite  in  irregular  splotches  (fig.  81),  or  along  crystallo- 
graphic directions  (faintly  shown  in  fig.  82). 

It  seems  probable,  as  one  of  us  85  has  stated,  that  a  first  generation 
of  chalcocite  (fig.  82)  was  formed  before  the  sericite  and  chlorite,  and 
a  second  generation  (fig.  83)  after  sericite  and  chlorite.  The  former  is 
hypogene,86  the  latter  supergene. 

84  Rogers,  A.   F. — The  so-called  graphic  intergrowth  of  bornite  and  chalco- 
cite.    Econ.  Geol.,  11,  582-593  (1916). 

85  Rogers,   A.   F. — Sericite   a   low-temperature   hydrothermal   mineral.     Econ. 
Geol.,  11,  118-150  (1916). 

86  Ransome,  F.  L. — Copper  deposits  near  Superior,  Arizona.     Bull.  540,  U.  S. 
Geol.  Surv.,  152  (1912). 


64  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

SUMMARY 

The  Engels  mine,  which  is  rapidly  becoming  one  of  the  important 
copper  mines  of  California,  is  unique  in  that  magmatic  bornite  is  the 
principal  ore-mineral.  The  ore  occurs  as  a  vertical  ore  shoot,  and  in  a 
massive  norite-diorite,  in  such  relation  to  the  enclosing  rock  that  only 
a  magmatic  origin  seems  probable.  Bornite  and  associated  chalcopyrite 
are  formed  by  mineralizers  at  a  late  magmatic  stage,  and  not  by  hydro- 
thermal  solutions.  This  is  proved  by  the  comparative  freedom  from 
alteration  in  many  of  the  ores  and  associated  rocks.  The  silicification 
accompanying  hydrothermal  deposits  generally  is  practically  absent. 
Locally  there  may  be  sericitization  and  chloritization,  but  the  bornite 
and  chalcopyrite  are  no  more  abundant  in  the  ores  thus  affected  than 
in  the  unaltered  ores.  The  secondary  copper  minerals,  covellite,  chal- 
cocite,  and  chalcopyrite  of  the  second  generation,  have  developed  at  a 
later  stage,  in  part  by  hypogene  and  in  part  by  supergene  solutions. 

BIBLIOGRAPHY  OF  THE  ENGELS  ORE  DEPOSIT 

Aubury,  L.  E. — The  copper  resources  of  California.  Bull.  no.  50,  California  State 
Mining  Bureau,  pp.  185-186  (1908). 

Turner,  H.  W.,  and  Rogers,  A.  F. — A  geologic  and  microscopic  study  of  a  mag- 
matic copper  sulphide  deposit  in  Plumas  county,  California,  and  its  modi- 
fication by  ascending  secondary  enrichment.  Econ.  Geol.,  9,  359-391  (1914). 

Read,  T.  T.— The  Engels  mine  and  mill.  Min.  and  Sci.  Press,  111,  167-171 
(1915). 


OTHER  DEPOSITS  CLASSIFIED  AS   MAGMATIC  65 


REMARKS    ON   CERTAIN    OTHER   DEPOSITS    THAT   HAVE 
BEEN  CLASSIFIED  AS  MAGMATIC 

PYRITIC  DEPOSITS 

Beyschlag,  Krusch,  and  Vogt 87  classify  certain  pyrite  ores  as  mag- 
matic  ores  under  the  heading  "Die  intrusiven  Kieslagerstatten."  Among 
these  they  place  the  Rio  Tinto,  Bodenmais,  Sain  Bel  and  Chessy,  Agordo, 
and  numerous  Norwegian  deposits.  It  is  certain  that  some  of  these 
pyritic  ores  are  hydrothermal  in  origin.  The  Rio  Tinto  deposits,  for 
example,  are  due  to  metasomatic  replacement  of  crushed  and  sheared 
zones  by  hydrothermal  solutions,  as  was  definitely  proved  by  Finlayson.88 
The  Rio  Tinto  and  other  pyritic  deposits,  such  as  the  Rammelsberg,  are 
placed  by  Lindgren  89  in  his  division  "Deposits  formed  at  Intermediate 
Depths."  Lindgren,  however,  recognizes  that  Some  of  the  pyritic 
deposits  may  be  due  to  the  injection  of  molten  sulfids. 

The  so-called  intrusive  or  injected  pyritic  ores  occur  for  the  most 
part  in  gneisses.  Igneous  rocks  are  often  closely  associated,  tho  not 
always.  The  igneous  rocks,  however,  are  altered  rocks,  such  as  saus- 
surite-gabbro.  The  ore-minerals  are  associated  with  typical  metamor- 
phic  minerals  such  as  cordierite,  andalusite,  anthophyllite,  and  chloritoid. 

As  these  ores  occur  in  areas  of  regional  metamorphism,  and  are 
usually  found  in  gneisses,  schists,  and  not  often  in  the  igneous  rocks 
themselves,  it  is  difficult  to  see  why  they  are  placed  with  the  magmatic 
ores.  It  seems  far  more  reasonable  to  classify  them  as  metamorphic 
ores,  as  Berg90  does.  We  have  not  had  an  opportunity  of  examining 
many  of  these  ores,  but  certain  facts  derived  from  our  study  of  the 
undoubted  magmatic  ores  leads  to  the  conclusion  that  these  are  not  of 
this  type,  or  at  least  that  the  burden  of  proof  rests  upon  any  one  who 
classifies  them  as  such. 

In  the  first  place,  we  seriously  doubt  whether  pyrite  is  a  charac- 
teristic magmatic  mineral.  In  our  present  study  we  have  not  found 
pyrite  in  any  of  the  typical  disseminated  magmatic  ores,  and  our  speci- 

87Loc.  cit,  1,  298  (1910). 

Ore    Deposits.      English    translation    by   Truscott,    1,   301    (1914)- 

88  Finlayson,   A.   M. — The   pyritic   deposits   of   Huelva,    Spain.     Econ.   Geol., 
5,  357-372,  403-437   (iQio). 

89  Mineral  Deposits,  602  et   seq.    (1913).  90  Loc.  cit.,  114. 


66  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

mens  include  representative  suites  of  most  of  the  important  deposits 
except  those  from  Norway.  It  is  true  that  pyrite  occurs  in  some  of  the 
Sudbury  mines;  we  have  examined  several  of  these  occurrences,  and 
have  found  the  pyrite  to  be  distinctly  later  than  the  nickel-copper  sulfids. 

Later  mineralization  seems  to  be  more  prominent  at  the  Worthing- 
ton  mine  than  at  any  other  locality  in  the  Sudbury  district.  Walker91 
says:  "It  is  difficult  to  avoid  the  conclusion  that  the  ores  as  they  are 
now  found  at  the  Worthington  mine  have  been  subject  to  rearrangement 
by  aqueous  agencies  since  the  solidification  of  the  rock  and  sulfids  from 
the  original  magma."  Pyrite  is  evidently  not  a  typical  mineral  at  Sud- 
bury. Browne,92  for  example,  says:  "Practically  speaking,  there  is  no 
pyrite,  marcasite,  or  any  other  sulphide  (he  has  previously  mentioned 
pyrrhotite,  chalcopyrite,  and  pentlandite)  found  in  the  great  ore  bodies." 

Pyrite  is  entirely  absent  in  the  ores  from  the  Engels  mine.  This 
might  be  attributed  to  lack  of  sufficient  iron  to  combine  with  sulfur; 
but  in  the  Namaqualand  ores  there  was  sufficient  iron  to  form  pyrrhotite, 
yet  pyrite  is  entirely  absent. 

As  a  matter  of  fact,  pyrrhotite  seems  to  take  the  place  of  pyrite  in 
the  magmatic  ores.  The  work  carried  on  93  at  the  Geophysical  Labora- 
tory, proving  that  pyrrhotite  is  the  iron  sulfid  stable  at  high  temperature, 
may  explain  this.  Pyrite  has  usually  been  considered  a  persistent  min- 
eral.94 Its  maximum  development,  however,  is  reached  in  hydrothermal 
deposits,  and  it  is  less  important  in  contact  deposits  and  in  those  formed 
at  low  temperatures  by  meteoric  water.  True,  pyrite  is  common  in  igneous 
rocks ;  but  in  the  great  majority  of  cases  it  has  been  introduced  by  hydro- 
thermal  solutions.  There  are  a  few  authentic  cases95  of  pyrite  in  fresh 
unaltered  igneous  rocks,  and  so  its  occurrence  as  a  magmatic  mineral 
can  not  be  absolutely  denied;  but  it  seems  certain  that  it  is  of  minor 
importance  as  a  magmatic  mineral. 

91  Walker,    T.    L. — Certain    mineral   occurrences    in    the    Worthington    mine, 
Sudbury,  Ontario,  and  their  significance.     Econ.  Geol.,  10,  542   (1915). 

92  Browne,  D.  H. — Notes  on  the  origin  of  the  Sudbury  ores.     Econ.  Geol.,  7, 
468  (1906). 

93  Allen,  E.  T.,  Crenshaw,  J.  L.,  and  Johnston,  J. — The  mineral  sulphides  of 
iron.    Am.  Jour.  Sci.  (4),  33,  169-236  (1912). 

94  Lindgren,    W. — The     relation    of    ore-deposition     to    physical     conditions. 
Econ.  Geol.,  2,  (1907). 

95  Lindgren,  W. — The   gold-quartz  veins   of   Nevada   City  and   Grass   Valley 
districts,  California.    i;th  Ann.  Kept.  U.  S.  Geol.  Surv.,  pt.  II,  66  (1896). 

Spurr,  J.  E.,  and  Carrey,  G.  H.— Prof.  Paper  no.  63,  U.  S.  Geol.  Surv.,  388 
(1908). 


OTHER  DEPOSITS  CLASSIFIED  AS  MAGMATIC  67 

MAGNETITE-ILMENITE  DEPOSITS 

Geologists  apparently  find  little  difficulty  in  accepting  the  titanifer- 
ous  magnetites  as  magmatic  deposits,  probably  because  magnetite  is 
a  common  accessory  mineral  of  igneous  rocks,  but  appear  to  be  more 
cautious  in  regard  to  the  sulfid  deposits.  We  have  examined  a  number 
of  examples  of  the  magmatic  iron  ores  in  "basic"  rocks,  and  find  that  the 
relations  of  the  ores  to  silicates  are  similar  in  all  respects  to  those  of 
the  sulfid  deposits.  In  the  latter  the  sulfids  may  fail  locally  and  the  ores 
become  identical  with  the  low-grade  titaniferous  iron  ores.  The  sulfid 
rich  and  sulfid  free  magnetite  deposits  alike  develop  subhedral  crystals 
within  the  silicates;  and  the  larger  anhedral  crystals  cut,  surround,  and 
replace  the  silicates.  They  are  often  unaccompanied  by  secondary  alter- 
ation products,  and  where  these  are  present,  they  are  later  than  the 
ores. 

In  order  to  check  our  incomplete  study,  we  have  examined  the  liter- 
ature of  these  deposits,  and  find,  contrary  to  the  general  impression,  that 
the  majority  of  those  who  have  made  a  careful  microscopic  study  of  the 
ores  conclude  that  they  are  later  than  the  silicates.  As  a  result  of  a 
similar  review  of  the  literature,  Lindgren  96  states :  "In  these  differen- 
tiated magmas  ilmenite  and  magnetite  have,  as  a  rule,  crystallized  after 
the  silicates."  Berg  97  notes :  "Die  grosseren  Erzmassen  sind  stets  An- 
haufungen  des  jungeren  Erzes,  umschliessen  also  einzelne  Krystalle  von 
Hypersthen,  Diallag,  Olivin,  und  Feldspat." 

OTHER  MAGMATIC  IRON  ORES 

It  is  not  our  purpose  to  take  up  the  remaining  types  of  iron  ores 
for  which  a  magmatic  origin  has  been  suggested,  the  origin  of  some  of 
which  is  obscure  on  account  of  the  metamorphism  they  have  undergone. 
In  general,  those  deposits  related  to  the  less  "basic"  rocks,  such  as  the 
Kiirunavaara  type,98  and  certain  of  the  Adirondack  ores,"  show  evidence 
of  increasing  activity  of  mineralizers,  with  the  occasional  development 
of  pneumatolytic  and  allied  minerals,  and  a  tendency  to  migrate  further 
out  from  the  mother  rock. 

In  general,  iron  oxids,  like  the  magmatic  sulfids,  appear  to  be  con- 
centrated, not  during  the  early  magmatic  stages,  but  during  the  later 
stages  and  under  the  influence  of  mineralizers. 


06  Mineral  Deposits,  749.  97  Loc.  cit.,  102. 

98  Igneous    rocks    and    iron    ores    of   Kiirunavaara,    Luossavaara,    and   Tuol- 
lavaara.     Econ.  Geol.,  5,  696-718   (1910). 

99  Newland,    D.    H. — On   the   association  and   origin   of  the   non-titaniferous 
magnetites  in  the  Adirondack  region,  2,  763-773  (1907). 


68  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

CHROMITE  DEPOSITS 

Of  the  remaining  ore-minerals  concentrated  as  magmatic  segrega- 
tions, nickel-iron,  gold,  and  platinum,  do  not  occur  in  sufficient  amounts 
to  be  considered  as  ores. 

Chromite  in  unaltered  rock  from  Norway,  has  been  described  by 
Vogt,100  and  is  considered  by  him  to  be  the  oldest  mineral  of  the  rock, 
on  account  of  its  occurrence  as  sharp  octahedra.  Examination  of  his 
figures  shows  irregular  as  well  as  euhedral  forms;  and  the  alternate 
hypothesis  that  the  chromite  octahedra  are  formed  at  a  late  stage,  is 
worthy  of  consideration. 

Vogt  states :  "The  chromite  deposits  in  peridotite  show  accordingly 
the  same  geological,  petrographical  and  morphological  characteristics  as 
those  of  titaniferous  iron  in  gabbro,  and  the  general  genetic  statements 
afterwards  enumerated  in  connection  with  the  titaniferous  iron  deposits 
hold  good  also  for  the  occurrence  of  chromite."  101 


100  Beitrage    zur   genetischen    Classification    der    durch    magmatische    Differ- 
entiationsprocesse   und   durch   Pneumatolyse   entstanden   Erzvorkommen.     Zeit.   f. 
prakt.  Geol.,  Jahrg.   1894,  PP-  384-393- 

101  Ore   Deposits :     Beyschlag,   Vogt,   and   Krusch ;   Translated   by   Truscott, 
(1914).  246. 


PART    III. 
SUMMARY    AND    CONCLUSIONS 


CRITERIA  FOR  THE  RECOGNITION  OF  MAGMATIC  ORES 

Any  discussion  of  the  criteria  by  which  the  magmatic  ores  may  be 
separated  from  other  types  of  ores  must  be  somewhat  tentative  in  nature, 
as  long  as  the  latter  have  not  received  detailed  comparative  study. 

The  fact  that  the  ores  migrate  only  a  short  distance,  if  at  all,  into 
the  adjoining  rock,  distinguishes  them  from  contact  deposits  formed 
chiefly  in  the  intruded  rock,  and  from  many  other  high-temperature 
deposits. 

Comparison,  then,  must  be  made  with  ores  that  occur  in,  and  as  an 
integral  part  of,  igneous  rocks. 

The  fact  that  the  only  high-temperature  alteration  mineral  present 
in  appreciable  amount  is  hornblende,  which  we  believe  is  formed  by  mag- 
matic processes,  and  that  tourmaline  and  garnet  are  only  occasionally 
developed,  distinguishes  these  deposits  from  those  in  which  destructive 
pneumatolytic  and  contact  action  is  prominent. 

The  fact  that  hydrothermal  alteration  of  the  silicates  is  often  minor 
in  amount,  is  invariably  later  than  the  ores,  and  generally  is  not  accom- 
panied by  the  deposition  or  migration  of  ores,  differentiates  these  ores 
from  those  of  hydrothermal  origin. 

The  absence  of  silicification  and  albitization,  and  the  only  occasional 
development  of  considerable  amounts  of  sericite,  separate  these  deposits 
from  moderate-  and  high-temperature  ores  in  igneous  rocks,  such  as  the 
copper  ores  of  Bingham  and  Butte  and  the  gold  ores  at  Treadwell, 
Alaska,  etc. 

There  is  definite  order  of  succession  of  the  magmatic  ore-minerals, 
as  follows:  magnetite,  hematite,  pyrrhotite,  pentlandite,  chalcopyrite, 
bornite.  Any  change  in  this  order  is  due  to  rearrangement  subsequent 
to  the  magmatic  period. 

Pyrite  is  not  a  typical  magmatic  mineral ;  if  present  in  a  magmatic 
ore,  it  is  introduced  later  than  the  magmatic  period. 


70  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 


SUMMARY  OF  THE  CHARACTERISTICS  OF  MAGMATIC 

ORES 

Contrary  to  general  opinion,  the  magmatic  sulfids  are  formed  by 
replacement  of  the  silicates  after  the  solidification  of  the  igneous  rock. 
Notwithstanding  this,  we  retain  the  term  "magmatic  ore  deposits"  for 
the  types  of  ores  described  in  this  paper,  because  they  have  a  close  genetic 
relation  to  the  intrusive  rock  in  which  they  occur,  and  because  they 
are  formed  within  the  magmatic  stage  as  defined  by  us.  Regardless  of 
any  theory  as  to  their  genesis,  however,  these  deposits  have  definite  and 
easily  recognizable  characteristics,  which  distinguish  them  from  all  other 
types  of  ore  deposits. 

The  characteristics  of  the  magmatic  sulfid  ores  as  brought  out  by 
our  study  may  be  summarized  as  follows : 

(1)  They  occur  in  subsilicic  rocks  of  the  norite,  gabbro,  peridotite, 
or  related  families. 

(2)  In  most  occurrences  the  containing  rock  is  either  dominantly 
subsilicic,  with  minor  amounts  of  complementary  persilicic  differentiates 
(Insizwa),  or  occurs  as  lenses  of  mafic  rock  in  a  large  granitic  intrusion. 
(Golden  Curry). 

(3)  The  subsilicic  rock  occurs  generally  as  small  dikes,  sills,  or 
stocks,  and  rarely  as  a  large  laccolith  (Sudbury). 

(4)  In  most  cases  the  ore-bearing  rock  has  undergone  marked  dif- 
ferentiation, and  the  differentiated  portions  are  sharply  separated  and 
do  not  grade  into  each  other  (Ookiep). 

(5)  The  ore  may  occur  in  any  variety  of  rock  produced  by  differ- 
entiation, but  in  any  one  locality  the  ore  shows  marked  preference  for 
certain  types  of  rock  and  occurs  sparingly  in  others  (Ookiep). 

(6)  Pegmatite  and  aplite  dikes  often  cut,  and  are  therefore  later 
than,  the  ore  bodies  (Erteli). 

(7)  The   ore   is   generally  segregated   at   the   margins   of  the   in- 
trusives,  but  occasionally  occurs  as  lenses  or  tabular  ore  shoots  well 
within  the  intrusive  magma    (Engels).     In  sills,  the  ore  is  usually  at 
the  base  of  the  intrusives  (Insizwa)  ;   in  dikes,  it  often  is  formed  along 
the  footwall  (Sohland),  or  as  columnar  or  irregular  shoots  occupying 
the  entire  width  of  the  dike  (Copper  Cliff). 

(8)  Very  often  the  ore  is  concentrated  in  those  portions  of  the  in- 
trusive which  have  suffered  brecciation  during  intrusion  (Sudbury). 


SUMMARY  AND  CONCLUSIONS  71 

(9)  The  ore  migrates  only  a  short  distance  into  the  adjacent  rock, 
from  a  few  inches  to  a  few  score  of  feet  at  most  (Sudbury). 

(10)  The  "offset"  deposits,  formed  in  dikes  at  some  distance  from 
the  main  intrusive,  are  accompanied  and  cut  by  veins  carrying  ore  and 
gangue  minerals  of  hydrothermal  origin  (Sudbury). 

(n)  There  is  no  essential  distinction  between  the  sulfid  group  and 
the  magnetite-ilmenite  group  as  to  the  origin  or  the  relation  of  the  ores 
to  the  silicate  minerals.  In  all  the  magmatic  ores  examined,  ore  dep- 
osition takes  place  at  the  close  of  the  magmatic  period. 

(12)  There  are  two  general  classes  of  the  magmatic  sulfid  ores: 
(a)  pyrrhotite-chalcopyrite  deposits,  and  (b)  chalcopyrite-bornite  depos- 
its. 

(13)  The  so-called  pyritic  intrusive  ores  are  not  magmatic. 

(14)  The   principal   ore-minerals   of  the  magmatic  period  include 
magnetite,  ilmenite,  hematite,  pyrrhotite,  pentlandite,  chalcopyrite,  and 
bornite 

(15)  Pyrrhotite  and  bornite  have  not  been  found  together  in  mag- 
matic ores. 

(16)  Pyrite  is  not  a  typical  magmatic  mineral. 

(17)  The  ore-minerals  are  formed  at  a  late  magmatic  stage  by  a 
partial  replacement  of  the  silicate  minerals.    The  ores  surround,  embay, 
and  cut  all  the  earlier  silicates.     They  penetrate  the  cleavable  minerals. 
They  occasionally  occur  as  sharp  veinlets  which  lead  out  from  the  larger 
sulfid  masses.     Selective  replacement  is  shown  by  the  preservation  in 
the  ores  of  an  original  graphic  texture  of  the  rock. 

(18)  There  is  a  definite  order  of  formation  of  the  principal  mag- 
matic ore-minerals.     This  order  is  as  follows:    magnetite-ilmenite   (in- 
tergrowth),  hematite,  pyrrhotite,  pentlandite,  chalcopyrite,  and  bornite. 

(19)  There  is  evidence  of  the  replacement  of  one  magmatic  ore- 
mineral  by  another. 

(20)  Euhedral  magnetite  and  probably  other  minor  accessories  oc- 
curring in  euhedral  crystals,  such  as  apatite,  zircon,  titanite,  etc.,  are  also 
formed  at  a  late  magmatic  stage. 

(21)  There  is  clear  evidence  of  the  magmatic  alteration  of  pyrox- 
ene to  hornblende  prior  to  the  introduction  of  the  ore-minerals. 

(22)  Hydrothermal  alteration,  altho  seldom  lacking  in  magmatic 
ores,  is  relatively  insignificant,  and  is  distinctly  later  than  the  magmatic 
ore  period.     The  silicates  of  the  hydrothermal  period  include  tremolite, 
anthophyllite,   sericite,   chlorite,   and  serpentine.     These  secondary  sili" 
cates  often  replace  the  magmatic  ore-minerals  in  veinlets  and  in  sharp 


72  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

lath-shaped  crystals  without  causing  any  change  or  migration  of  the 
ore-minerals. 

(23)  The  attention  given  these  alteration  products  in  order  to  de- 
termine their  relative  age,  may  have  given  an  erroneous  impression  as 
to  their  relative  abundance.    In  most  of  the  ores  studied  they  are  present 
only  in  minor  amounts.     Many  of  the  magmatic  ores  are  as  free  from 
alteration  products  as  the  average  unmineralized  igneous  rock. 

(24)  However,  a  minor  amount  of  rearrangement,  consisting  in  the 
production  of  microscopic  crystals  of  pentlandite  and  chalcopyrite  of  the 
second  generation,  has  been  detected  in  the  pyrrhotitic  ores.     In  the 
chalcopyrite-bornite  ores*  there  has  been  some  migration,  resulting  in  the 
formation  of  minor  amounts  of  covellite,  chalcocite,  and  chalcopyrite  of 
the  second  generation.     This  alteration  is  only  prominent  where  there 
has  been  an  abnormal  development  of  sericitization. 

(25)  The  role  of  mineralizers  in  magmatic  differentiation  has  not 
been  sufficiently  emphasized.     The  crystallization  of  the  early  formed 
minerals    in   the   magma   involves   the   complementary   process    of   the 
"squeezing  out"  of  the  residual  fluid.    This  process  is  not  merely  a  me- 
chanical one,  but  is  also  due  to  gaseous  extraction. 

(26)  The  typical  magmatic  deposits,  unaccompanied  by  high-tem- 
perature  alteration,   with   the  exception   of   magmatic   hornblende,   are 
chiefly  developed  in  "basic"  rocks.     Ore  deposits  genetically  related  to 
persilicic  rocks  show  intense  rock  alteration,  probably  the  result  of  min- 
eralizers more  "active"  than  those  accompanying  the  subsilicic  rocks. 

(27)  There  is  a  parallelism  between  the  various  groups  of  high- 
temperature  deposits,  of  which  the  magmatic  ores  are  one  division.     In 
all  groups,  high-temperature  silicates  precede  the  introduction  of  the  ore, 
and  hydrothermal  stages  follow.    In  contrast  with  the  magmatic  deposits, 
the  non-magmatic  ores  are  characterized  by  a  complex  set  of  pre-mineral 
silicates,  and  the  hydrothermal  stage  is  generally  the  most  important 
period  of  ore  introduction. 

(28)  The  temperature  at  which  the  introduction  of  ore-minerals 
is  initiated  is   about  the  same   for  all  the  high-temperature   deposits; 
probably  not  higher  than  300-400°  C. 

(29)  Gradations  between  the  typical  magmatic  ores  and  other  high- 
temperature  deposits  are  shown  by  the  local  development  of  garnet  and 
tourmaline.     Further,  the  gradations  of  these  into  intermediate-temper- 
ature and  low-temperature  deposits  is  strong  evidence  of  the  magmatic 
origin  of  ore  deposits  in  general. 


SUMMARY  AND  CONCLUSIONS  73 

(30)   The  following  orderly  series  of  events  is  recognized  in  mag- 
matic  deposits : 

(a)  Crystallization  of  primary  silicates; 

(b)  The  development  of  hornblende  and  biotite,  and  occasionally 
tourmaline  and  garnet,  as  magmatic  alteration  products; 

(c)  The  introduction  of  the  ore-minerals; 

(d)  A  small  amount  of  rearrangement  of  the  ores  and  the  devel- 
opment of  secondary  silicates  by  hydrothermal  solutions. 

Department  of  Geology, 

Leland  Stanford  Junior  University, 

October,  1916 


74 


A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 


EXPLANATION  OF  PLATES  AND  METHODS  OF  PREPARING 
PHOTOGRAPHS  AND  SECTIONS 

The  photographs  are  of  two  types:  thin  sections  and  polished  sec- 
tions. The  thin  sections  are  necessary  for  the  identification  of  the  sili- 
cates; the  polished  sections  for  the  identification  of  the  ore-minerals, 


PLAN 

App<mnt  thickness  <*x 
ft  toy 


Fig.  6A.  Drawn  from  a  photograph  of  a  polished  thin 
section  of  ore  from  the  Stobie  mine,  Sudbury.  White 
areas  are  silicates.  Lined  area  is  opaque  band  as 
viewed  by  transmitted  light.  Black  band  is  veinlet  as 
shown  by  reflected  light  (x  175) 

and  to  show  the  relations  of  the  various  minerals  to  each  other.  In  ex- 
amining the  photographs  note  that  the  ore-minerals  are  black  and  the 
silicates  light  in  thin  sections,  while  the  silicates  are  dark  gray  and  the 
ore-minerals  white  or  light  gray  in  the  polished  sections. 


PLATES,   PHOTOGRAPHS   AND   SECTIONS  75 

In  taking  the  photographs  we  have  used  Wratten  panchromatic 
plates,  selecting  in  each  case  the  color  screen  best  adapted  to  bring  out 
all  the  minerals  in  the  polished  section. 

In  the  microscopic  investigation  we  examined  ( I )  thin  sections  with 
the  polarizing  microscope;  (2)  polished  sections  with  the  reflecting 
microscope;  and  (3)  polished  thin  sections  (combination  sections)  with 
both  transmitted  and  reflected  light. 

It  is  now  generally  admitted  that  the  examination  and  determination 
of  the  opaque  minerals  in  thin  sections  is  at  best  unsatisfactory,  and  that 
only  the  grosser  relations  of  the  opaque  minerals  to  each  other  can  be 
made  out,  and  that  those  details  requiring  high  magnifications  are  lost 
entirely. 

In  addition  to  the  above,  we  have  found  that  the  relations  of  the 
opaque  minerals,  as  a  group,  to  the  transparent  minerals,  are  shown  more 
accurately  in  polished  surfaces  than  in  thin  sections,  and  that  the  rela- 
tions of  the  transparent  silicates  to  each  other  are  sharper  when  viewed 
with  the  reflecting  microscope  than  with  the  polarizing  microscope,  altho 
the  identification  of  the  transparent  minerals  must  be  made  with  the 
latter. 

The  reason  for  this  is  simple.  In  examining  a  thin  section,  one 
looks  thru  a  certain  thickness  of  rock,  and  not  at  a  single  definite  sur- 
face. All  contacts  between  the  opaque  and  transparent  minerals  that  are 
not  perpendicular  to  the  surface  of  the  slide  are  widened  so  that  the 
black  area  represents  the  projection  on  the  surface  of  the  section  of  all 
the  opaque  material  contained  in  the  slide.  A  thin  section  does  not  show, 
therefore,  the  relation  of  the  opaque  and  the  transparent  minerals  at  one 
definite  level  as  do  the  polished  sections.  Fig.  6a,  page  74,  was  drawn 
from  a  photograph  of  a  polished  thin  section  viewed  under  both  trans- 
mitted and  reflected  light.  The  ruled  band  represents  the  shadow  cast 
by  the  opaque  material  as  viewed  with  transmitted  light,  and  the  thin 
black  line  at  the  margin  of  the  band  shows  the  true  thickness  of  the 
veinlet  as  registered  by  the  light  reflected  from  the  polished  surface. 

In  order  to  take  advantage  of  the  reflecting  microscope  for  this 
work,  it  was  necessary  to  develop  special  methods  of  polishing  in  order 
to  give  a  satisfactory  brilliant  surface  to  the  silicates  as  well  as  to  the 
hard  and  soft  opaque  minerals.  As  a  matter  of  fact,  the  polishing  of  all 
the  different  constituents  in  a  single  section  of  complex  ore  is  an  art, 
and,  up  to  date,  only  crude  methods  of  polishing  have  been  described  in 
the  literature. 


76  A  STUDY  OF  THE  MAGMATIC  SULFID  ORES 

Our  methods,  in  brief,  consist  in  grinding  on  a  glass  plate  a  plane 
surface  with  the  minimum  of  pits,  using  successively  finer  grades  of 
carborundum,  and  then  "sixty-minute"  carborundum,  and  finally  a  still 
finer  carborundum  prepared  in  our  laboratory. 

The  final  polishing  is  done  with  fine  cambric  on  rapidly  rotating 
wheels  (1700-3400  r. p.m.).  Silicates,  magnetite-ilmenite,  pyrite,  etc.,  are 
polished  by  carborundum,  prepared  by  grinding  the  "sixty-minute"  grade 
two  weeks  in  a  ball  mill,  and  then  floating  off  the  finest  product  for  this 
purpose.  Chrome  oxid  is  used  to  polish  sulfids  of  average  hardness,  and 
aluminum  oxid  for  the  softer  sulfids.  These  powders  are  applied  one 
after  the  other,  or  as  mixtures,  depending  upon  the  character  of  the 
specimen.  To  develop  a  polish  of  the  highest  brilliancy  on  the  sulfids — 
for  example  to  bring  out  pentlandite  enclosed  in  pyrrhotite — aluminum 
oxid  is  used  on  the  finest  billiard  felt. 

In  most  cases  we  find  it  satisfactory  to  use  one  side  of  a  sawed  speci- 
men for  the  polished  section  and  the  adjacent  surface  for  the  thin  sec- 
tion. In  some  cases  a  thin  section  is  given  a  high  polish,  which  acts  like 
a  cover-glass  in  giving  clear  images  with  transmitted  light,  and  also 
gives  brilliant  reflections  with  reflected  light. 


PLATE  I 
PHOTOGRAPHS  OF  POLISHED  SPECIMENS  OF  SUDBURY  ORES 

FIG.  7 

Rich  ore  (pyrrhotite  and  pentland- 
ite)  from  Copper  Cliff  mine,  show- 
ing1 unreplaced  silicates  fdark  areas] 
of  the  granitic  rock. 

Nat.  size 


FIG.  8  FIG.  9 

Ore     from     the     Creighton     mine  Xodule     of     "basic"     material     in 

showing   chalcopyrite   veinlets.  granitic  rock  which  has  been  largely 

x  i  2/3  diameters  replaced   by   pyrrhotite    [light   gray]. 

Creighton  mine. 

x  87/100 


FIG.  10 

Pyrrhotite  flight  gray]  impregnat- 
ing and  replacing  "greenstone  schist." 
Garson  mine. 

x  8/10 


Fig.  10 


PLATE  II 

PHOTOMICROGRAPHS  OF  THIN  SECTIONS  OF  LEAN  ORE  FROM  THE  STOBIE 

MINE,  SUDBURY 

FIG.   ii  FIG.  12 

Typical  norite  with  euhedral  mag-  Ore-minerals ;    magnetite,    pyrrho 

netite     [black],    hypersthene     [gray],  tite   [black],  surrounding  and  rephic- 

plagioclase    [white].  ing    hypersthene    [gray].      A    vcinlet 

Magnification  x  36  diameters.  of  chlorite    [ch]   cuts  magnetite.     In 

the  upper  portion  of  the  photograph 
the  hypersthene  is  fresh,  and  in  the 
lower  part  it  is  altered ;  and  yet  the 
ore  shows  the  same  relation  to  the 
silicates  in  the  fresh  and  altered  por- 
tions. 

x  30 


FIG.   13  FIG.   14 

A  veinlet  of  chlorite   [ch],  cutting  Tremolite     [/;•]    crystals    cutting— 

the  sulrids,  chalcopyrite  and  pyrrho-  not  residual  in— chalcopyrite  [black  1. 

tite  [black],  and  the  silicates.  and   projecting  from  altered  hypers- 

x  36  thene    [gray]. 

x  119 


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Fig.  ii 


Fig.  12 


•ite^-^  £ 

^m^ 


Fig.  13 


Fig.  14 


PLATE  III 

PHOTOMICROGRAPHS  OF  POLISHED  SECTIONS  OF  ORES  FROM   THE  STOBIE 

MINE,  SUDBURY 

FIG.  15  FIG.  1 6 

Lean    ore.      Chlorite    veinlets    cut-  Plagioclase     [dark],    uralite    [light 

ting  ore-minerals ;  magnetite,  pyrrho-  gray],  and  sulfids   [white].     Figs.   17 

tite,  chalcopyrite  [light],  and  the  sili-  and     18     are     higher     magnifications 

cates.  from  the  same   field, 

x  ii  x  10 


FIG.   17  FIG.   1 8 

Veinlet    and    crystals    of    tremolite  Pyrrhotite      [/>]      and     pentlandite 

[gray]    cutting  pyrrhotite.  I/*"]]  cut  by  irregular  veinlets  of  sec- 

x  270  ondary  silicates  [ss].    A  second  gen- 

eration of  pentlandite  (?)  [/w]2 
grows  out  in  tufts  from  these  vein- 
lets.  The  secondary  migration  of 
ore-minerals  is  therefore  insignificant 
in  amount. 

Aggregates  of  tremolite  crystals  on 
the  right  of  the  photograph. 
x  165 


Fig.  15 


Fig.  16 


Fig.  17 


Fig.  18 


PLATE  IV 
PHOTOMICROGRAPHS  OF  ORES  FROM  SUDBURY 

FIG.  19  FIG.  20 

Polished  section  of  rich  ore  from  Copper    Cliff    mine.      Thin    section 

the    Stobie    mine.      Irregular    areas  of    ore    in    coarse    granitic    material. 

and  veinlets  of  chalcopyrite    [white]  Ore-minerals    [black   areas],   magne- 

replacing  garnet    [g]   and  other   sili-  tite   and   pyrrhotite,   cutting   and   re- 

cates.  placing  biotite. 

x  16  x  38 


FIG.  21  FIG.  22 

Polished    section,    Creighton    mine.  Polished    section,    Creighton    mine. 

Crystallographic  intergrowth  of  mag-  Typical   rich   ore   showing  pyrrhotite 

netite    and    ilmenite     [parallel     dark  |/>]    with    veins    of   pentlandite    [pn] 

lines].     Specimen  is  not  etched,  but  and   residual  magnetite    [m]   and  sil- 

contrast     is     brought     out    by     high  icates    \s]. 

polish.  x  14 
x  700 


Fig.  19 


Fig.  20 


Fig.  21 


Fig.  22 


PLATE  V 

I'lloTo.MK 'KOdRArilS  OF  THIN  SECTIONS  OF  ORE  FROM   SUDBURY 


FIG.  23 

Ore  from  the  Creighton  mine. 
Large,  irregular  ore  masses  [black] 
are  chalcopyrite  and  pyrrhotite. 
Small  subhedral  black  spots  are 
chalcopyrite,  probably  a  replacement 
of  magnetite,  as  they  contain  re- 
sidual specks  of  the  latter.  .  Ore- 
minerals  cut  through  the  silicates 
and  replace  them.  Veinlets  at  the 
bottom  of  photograph  are  chalco- 
pyrite. 

x  28 


FIG.  24 

Creighton       mine.         Chalcopyrite 
veinlets       [black]       cut      microcline, 
quartz,  and  biotite  [dark  gray],  which 
are  free  from  alteration  products, 
x  66 


FIG.  25 

Creighton  mine.  Sulfid  veinlets, 
chalcopyrite  and  pyrrhotite,  which 
extend  out  from  a  mass  of  sulfids 
[black],  cut  silicates  [hornblende 
and  plagioclase]. 

x  27 


FIG.  26 

Evans  mine.  Chalcopyrite  veinlet 
cuts  hornblende  [ho],  biotite  [bi], 
and  plagioclase  [white].  Displace- 
ment, shown  by  hornblende  crystal, 
has  taken  place  along  this  veinlet. 
Beyond  the  field  of  the  photo- 
graph chlorite  cuts  across  the  veinlet. 
[ap]  apatite. 

x  158 


Fig.  23 


Fig.  24 


Fig.  25 


Fig.  26 


PLATE  VI 

PHOTOMICROGRAPHS  OF  POLISHED  SECTIONS  OF  "ACID"  ORE-BEARING  ROCK 
FROM  THE  CREIGHTON  MINE,  SUDBURY 

FIG.  27  FIG.  28 

Ore    masses,    pyrrhotite    and    chal-  Sulfids,    pyrrhotite    [/>]    and    chal- 

copyrite,   extend   out   into   fine   vein-  copy  rite   [cp]  cut  magnetite   [m]   and 

lets,      chiefly      chalcopyrite.        Large  silicates    in    anastomosing   veinlets. 

masses    replace    the    silicates.      The  x  15 
veinlets    cut    the    subgraphic     inter- 
growth    of   quartz    [q]    and   feldspar 

m 

X   IO 


FIG.  29  FIG.  30 

Chalcopyrite    [cp]    extending   from  Sharp  veinlets  of  chalcopyrite   cut 

the  main   mass   into  veinlets  as   one  the    graphic    intergrowth    of    quartz 

generation   of  ore.      No  evidence   of  [dark]    and    feldspar    [light], 

"rearrangement"    or    second    genera-  x  260 

tion  of  ore  in  veinlets.  A  mass  of 
pentlandite  \f>n]  enclosed  in  chal- 
copyrite. 

x  99 


Fig.  27 


Fig.  28 


Fig.  29 


Fig.  30 


PLATE  VII 

PHOTOMICROGRAPHS  OF  POLISHED  SECTIONS  OF  "ACID"  ORE-JJK AKIX<;  ROCK 
FROM  THE  CREIGHTON  MINE,  SUDBURY 

FIG.  31  FIG.  32 

Selective     replacement    by     sulfids  Sulfids,    pyrrhotite    [/>]    and    chal- 

[ white]    of   feldspar    [light   gray]    in  copyrite     [cp]     and    veinlike    masses 

the       feldspar-quartz        intergrowth.  of    pentlandite    [pn].      In   the   center 

Quartz  is  dark  gray.  of    the    field    chalcopyrite    preserves 

x  18  the    structure    of    the    graphic    inter- 

growth    by    selective    replacement    of 
the  feldspar. 

x  18 


FIG.  33  FIG.  34 

Graphic  intergrowth  of  quartz  and  An  irregular  area  of  magnetite  re- 

feldspar.     A  small  irregular  replace-  placing  silicates. 

ment    veinlet    cuts    the    fine    graphic  x  140 

intergrowth  above.  Large-scale 
graphic  intergrowth  below  preserved 
by  selective  replacement  of  feldspar 
by  sulfids. 

x  71 


Fig.  33 


Fig.  34 


PLATE  VIII 

I'lioTOMK  KOCKAI'KS  OF    POLISHED   SECTIONS    OF   ORES    FROM    SUDBURY 


Massive  ore  from  the  Creighton 
mine  showing  typical  vein-like  areas 
of  pcntlandite  [/>»],  in  pyrrhotite 
|/»|.  Contrast  brought  out  by  pol- 
ishing, not  by  etching.  Tiny  brush- 
like  crystals  of  a  second  generation 
of  pentlandite  [/>«!.,  develop  along 
veinlets  and  contacts.  See  Fig.  36. 
x  18 


FIG.   36 

Massive  ore  from  the  Creighton 
mine.  Second  generation  of  penthnd- 
ite  [fin]  along  a  veinlet  of  chalco- 
pyrite  [r/>],  cutting  pyrrhotite  [/>]. 
This  rearrangement  of  ore-minerals 
is  minor  in  amount,  and  can  rarely 
be  noted  even  under  high  powers. 
(  Same  specimen  as  shown  in  Fig. 

35-) 

x  670 


FIG.  37 

Massive  ore  from  the  Vermilion 
mine,  showing  pyrite  veinlets  [dark 
gray]  cutting  polydymite  ( ?)  [/>o] 
and  chalcopyrite  [cp].  Note  that 
pyrite  is  definitely  later  than  the 
typical  magmatic  ore-minerals  in  all 
the  occurrences  figured. 
x  10 


FIG.  38 

Ore    from    the    Worthington   mine. 
Pyrite     [white]      with     a     reticulate 
structure,     cutting     gangue      [black] 
and   sphalerite    [gray], 
x  58 


Fig.  35 


Fig.  37 


Fig.  38 


PLATE  IX 

I'llOTOMK  ROCRAPHS  OF  POLISHED  SECTION  OF  ORE  FROM  TIIK  Al.EXO  .MIX)- 
NORTHERN  ONTARIO,  CANADA 


FIG.  39 

Shows  the  general  relations  of  the 
ore-minerals  [large  white  areas]  to 
the  serpentinized  silicates.  The  orig- 
inal ore-minerals,  chiefly  pyrrhotite 
and  pentlandite  with  subordinate 
chalcopyrite  and  magnetite,  surround 
and  embay  the  serpentine  pseudo- 
morphs  after  olivine. 

A    minor    amount    of    the    second 
generation   of   chalcopyrite   has   been 
concentrated       by       serpentinization 
within    the    silicates. 
x  17 


FIG.  40 

Enlargement  of  a  part  of  the  field 
in  the  upper  left-hand  portion  of  Fif^. 
39.  Shows  serpentine  pseudomor- 
phous  after  olivine.  surrounded  by 
magmatic  sulfids,  the  contacts  of 
which  arc  not  modified  by  serpen- 
tinixation.  The  latter,  however,  af- 
fects a  secondary  concentration  of 
chalcopyrite  developed  as  a  border 
betwccn  two  different  types  of  ser- 
pentine. 

x  70 


Fig.  39 


Fig.  40 


PLATE  X 

PHOTOMICROGRAPHS  OF  OKI-:   FROM   THE  ALEXO  MINE,  FROM  THE  SAME 
1'OLISiIEI)  SECTION  FIGURED  IN  PLATE  IX 


KM;.  41 

Pyrrhotitc     |/>|,    pentlandite     \pn], 
chalcopyrite     !<"/>],•     ar|d     magnetite 
I///]   cut  by  veinlets  of  serpentine, 
x  44 


FIG.  42 

First  [pn]l  and  second  [pn]2  gen- 
erations of  pentlandite  in  pyrrho- 
tite  [/>].  The  two  generations  can 
be  distinguished  by  a  slight  differ- 
ence in  relief  and  color.  Serpentine 
veinlets  are  later  than  the  first  gen- 
eration pentlandite  and  closely  con- 
nected with  the  second  generation. 
Pentlandite  and  serpentine  tend  to 
develop  along  the  crystallographic 
lines  of  the  pyrrhotite. 
x  175 


FIG.  43 

Pyrrhotite  |/>]  and  pentlandite 
|/>»]  of  the  first  generation,  cut  by 
a  serpentine  veinlet  with  a  center  of 
magnetite  [m],  along  which  pent- 
landite of  the  second  generation 
I />»].,  develops. 

x  6ro 


FIG.  44 

Showing  later  generations  of  chal- 
copyrite [cp]0  connected  with  dif- 
ferent stages  of  serpentinization.  A 
large  mass  of  pyrrhotite  [p]  is 
shown  near  the  top  of  the  photo- 
graph. Small  dots  in  center  may  be 
a  third  generation  of  chalcopyrite. 
x  183 


Fig.  41 


Fig.  42 


Fig.  43 


Fig.  44 


PLATE  XI 

PHOTOMICROGRAPHS  OF  THIN   SECTION  OF  PYRRHOTITE-BEARING  PYROX- 
ENITE  FROM  THE  GOLDEN  CURRY  MINE,  ELKIIORN,  MONTANA 

FIG.  45 

Sulfids  [black],  chiefly  pyrrhotite 
with  some  chalcopyrite,  surrounding 
and  penetrating  augite.  White  need- 
les in  the  sulfids  in  the  center  of 
photograph  are  tremolite  and  darker 
patches  to  the  right  are  hornblende. 

X   22 


FIG.  46  FIG.  47 

Sulfids  replacing  hornblende  along  Sulfids   surrounding   and   penetrat- 

cleavages.  ing  pyroxene.     A  hornblende  crystal 

[The   white  streak    (lower  center)  [ho]    in    the    center    is    bordered    by 

is  a  crack  in  the  slide.]  tremolite   [tr]   needles  which  cut  the 

x  130  ore-minerals. 

x  90 


Fig.  46 


Fig.  47 


PLATE  XII 

PHOTOMICRORGAPHS  OF  POLISHED  SECTIONS  FROM   EVJE,  NORWAY.   AND 

SOHLAND,  GERMANY 

FIG.  48  FIG.  49 

Ore    from    the    Flaacl    mine,    Evje,  Ore    from   the    Flaad    mine,    Evje, 

N'orway.  Xorway. 

Pyrite    |/>y|    veinlet    and    detached  Pyrrhotite    [/>]   cut  by  chalcopyrite 

crystals  in  pyrrhotite    [/>]    and   mag-  \cf>]   which   in  turn  is   cut  by  pyrite 

netite  [»«].    The  veinlet  is  connected  [Py]    veinlets.      The     latter     connect 

with  alteration  products.     See  fig.  49  with    subhedral    and    euhedral    crys- 

for  the  relation  of  pyrite  crystals  to  tals.      The      euhedral      crystals      are 

pyrite  veinlets.  shown  in  fig.  48. 

x  12  x  300 


FIG.  50  FIG.  51 

Ore   from   Sohland,  Saxony.  Ore  from  Sohland,  Saxony. 

Shows  general  relations.     The  ore-  Uralite  [u]  needles  cutting  pyrrho- 

mincrals    are    pyrrhotite     [/>],    chal-  tite    [/>]    and   pentlandite    |>H].     The 

copyritc    |r/>],   and   pentlandite    [/>«].  uralite  (or  tremclite)  develops  at  the 

I'ralite  needles  \u]  penetrate  into  the  ends  of  the  hornblende  crystals, 

sullids.     Other  silicates  \s]  are  horn-  x  116 

blende  and  alteration  products.  A 
good  example  of  extensive  post-min- 
eral alteration. 

x  12 


Fig.  48 


Fig.  49 


Fig.  50 


Fig.  51 


PLATE  XIII 

PHOTOMICROGRAPHS  OF  SECTIONS  OF  ORE-BEARING  HYPERSTHENITE  FROM 
NABABEEP  MINE,  OOKIEP,  SOUTH  AFRICA 


FIG.  52 

Thin  section  showing  ore-minerals 
[black],  magnetite  and  a  little  born- 
ite,  cutting  and  surrounding  hy- 
persthene. 

x  74 


FIG.  53 

Thin  section  of  hypersthenite 
showing  euhedral  crystals  of  mag- 
netite within  the  hypersthene  and 
anhedral  ore-minerals,  chiefly  mag- 
netite, connected  with  the  "acid  ex- 
tract" [white].  The  "acid  extract" 
is  plagioclase. 

x  59 


FIG.  54 

Polished  section  of  ore-bearing 
hypersthenite.  The  ore-minerals  are 
magnetite  \m]  and  bornite  [b].  The 
general  relations  suggest  that  mag- 
netite is  partially  replaced  by  bornite. 
x  10 


FIG.  55 

Thin  section  showing  anthophyllite 
Jaw]  and  talc  (?)  [ta]  in  pyrrhotite 
•[black].  The  sharp-pointed  antho- 
fdiyllite  needles  cut  and  are  later 
titan  the  ore-minerals. 
x  435 


Fig.  52 


Fig-  53 


. 


Fig.  54 


Fig.  55 


PLATE  XIV 

PHOTOMICROGRAPHS  OF  THIN   SECTIONS   OF  ORE-BEARING   NORITE   FROM 
THE  TWEEFONTEIN  MINE,  OOKIEP,  SOUTH  AFRICA 

FIG.  56  .  FIG.  57 

I  lypersthene      (dark      gray]      and  A     higher     magnification     of     the 

plagioclase    [light    gray]    surrounded  lower  portion   of  fig.  56.     Magnetite 

and    cut    by    ore-minerals,    magnetite  is  anhedral  along  the  boundaries   of 

and   a    little   chalcopyrite.      Euhedral  the    silicates    and    euhedral    [see    ar- 

nia.L'iiotite    [designated   by   arrow]    is  row]    within  the  plagioclase. 

developed  within  the  silicates.     Sili  x  _|') 
cates  are  free  from  any  kind  of  al- 
teration products. 

x  17 


FIG.  58  FIG.  59 

Hypersthene     [gray]     and     plagio-  A     higher     magnification     of     the 

clase    [white]     surrounded     and     re-  plagioclase  in  the  left  side  of  fig.  58. 

placed    by    ore-minerals.      Anhedral  The     ore-minerals     are     "eating    in" 

arras    are    chalcopyrite    and    bornite.  along  albite  twinning  lamellae. 

Subhedral    crystals    within    the    hy-  x  23 
persthene  are  magnetite. 
x  II 


Fig.  56 


Fig.  57 


Fig.  58 


Fig.  59 


PLATE  XV 


PHOTOMICROGRAPHS  OF  A  POLISHED  SECTION    OF    ORE-BEARING 
TWEEFONTEIN  MINE,  OOKIEP,  SOUTH  AFRICA 


XORITK, 


I«"K;.  60 

The  ore-minerals  shown  in  the 
slide  are  bornite  [b],  chalcopyrite 
[cp],  magnetite  \m],  lined  with  il- 
menite.  (Hematite  is  shown  in  fig. 
61.)  Silicates  are  plagioclase  [pi], 
hypersthene  [hy],  biotite  [bi],  and 
apatie  [ap].  Note  especially  magne- 
tite cutting  hypersthene  in  the  upper 
portion  of  the  photograph,  and  born- 
ite and  chalcopyrite  penetrating  bio- 
tite on  the  left. 

x  9 


FIG.  6 1 

Photograph  from  the  same  polished 
section  as  shown  in  fig.  60,  showing 
hematite   [h],  and  magnetite  [m]    in- 
tergrown    with    ilmenite    lamellae. 
x  18 


FIG.  62 

Photograph  from  the  same  section 
as  shown  in  figs.  60  and  61.  Chalco- 
pyrite k/>]2,  and  bornite  [b]  cut  by  a 
veinlet  of  anthopyllite  [black  need- 
les], along  which  is  developed  a  sec- 
ond generation  of  chalcopyrite 
(shown  faintly  at  [c/>]0). 
x  150 


FIG.  63 

A  higher  magnification  of  the 
veinlet  shown  in  fig.  62.  Bornite 
[gray],  anthophyllite  [black],  and 
chalcopyrite  of  the  second  genera- 
tion [white]. 

x  920 


Fig.  60 


Fig.  61 


Fig.  62 


Fig.  63 


PLATE  XVI 

PHOTOMICKOCKAIMIS  OK  SKCTIONS  OF  ORE-BEARIXC  MICA  DIORITE,  ()OKIKI> 
EAST  MINE,  OOKIEP,  SOUTH  AFRICA 

FIG.  64  FIG.  65 

Polished    section    showing    pyrrho-  Thin     section.       The     ore-minerals 

tite  [/>]   residual  in  chalcopyrite   [r/>]  |  black]    are    pyrrhotite    and    chalco- 

and   sultid  veinlets  in  the  silicates.  pyrite.        The      primary      silicate      is 

x  10  plagioclase     [gray].     The    secondary 

silicates  cut  the  ore-minerals  and 
plagioclase  as  veinlets  in  the  lower 
portion  of  photograph,  and  are  dis- 
tributed as  specks  in  the  sulfids.  The 
alteration  is  subsequent  to  ore-form- 
ation. 

x  18 


FIG.  66  FIG.  67 

From  the  same  thin  section  as  fig.  A    group    of    subhedral    magiu-tiie 

65.     The  veinlets   of  sulfids  are  not  crystals    |m|,    a    few    of    which    an- 

a    later   generation    of    ore,    nor    are  partially    replaced   by   pyrrhotite    I/1] 

they    due    to    rearrangements.      This  [at   the   bottom    of   the   photograph], 

fact  is  shown  in  fig.  69.  x  51 
x  45 


Fig.  64 


Fig.  65 


Fig.  66 


Fig.  67 


PLATE  XVII 

PHOTOMICROGRAPHS  OF  POLISHED  SECTIONS  OF  ORE-BEARING  MICA  DIOR  in:, 
OOKIEP  EAST  MINE,  OOKIEP,  SOUTH  AFRICA 

FIG.  68  FIG.  69 

Pyrrhotite     [/>]     and     chalcopyrite  The    chalcopyrite     veinlet    on     the 

|r/>l    cutting   the    feldspars   in   sharp  right  is  cut  sharply  by  chloritic  alter- 

vcinlets  and  penetrating  the  cleavage  ation     products,    and     therefore     the 

planes  of  the  biotite    [top  of  photo-  veinlets   are   not   connected  with  the 

rock  alteration,  but  antedate  the  lat- 

x  10  ter.    On  the  left  a  biotite  crystal  [bi] 

is    penetrated    by    chalcopyrite    along 
cleavage  planes. 

x  153 


FIG.  70  FIG.  71 

A    biotite-rich    segregation    in    the  Brush-like    pentlandite     (?)     [/>»]2 

diorite,  with  the  sulfids,  chalcopyrite  replacing  pyrrhotite   [/>]   and  chalco- 

[cf>]   and  pyrrhotite   [/>],  penetrating  pyrite  \cp].     It  is  probably  of  a  late 

the  cleavages  of  the  biotite.  generation,   like   that   shown   in   figs. 

x  II  18,  35,  36,  42  and  43. 

x  620 


Fig.  68 


Fig.  69 


Fig.  70 


Fig.  71 


PLATE  XVIII 

PHOTOMICROGRAPHS   OF   THIN   SECTIONS   OF    NORITE-DIORITE   FROM    THE 
ENGELS  MINE,  PLUM  AS  COUNTY,  CALIFORNIA 


FIG.  72 

Unaltered  norite-diorite  showing 
the  general  relations  of  the  ore-min- 
erals [black]  to  the  silicates  (hypers- 
thene  fdark  gray]  and  feldspar  [light 
gray]).  The  ore-minerals  are  chiefly 
magnetite  and  hematite.  The  mag- 
netite shows  all  gradations  in  form 
from  euhedral  to  anhedral  outlines. 

X   22 


Fir,  73 

A  higher  magnification  of  a  spot 
in  the  upper  right-hand  corner  of 
fig.  72.  Euhedral  magnetite  occurs 
within  the  silicates,  anhedral  along 
the  boundaries.  One  area  [indicated 
by  arrow]  shows  an  euhedral  termi- 
nation penetrating  a  silicate  crystal. 
and  the  remaining  portion  is  an- 
hedral. 

x  143 


FIG.  74 

A  higher  magnification  of  another 
in  the  upper  right  corner  of  fig. 
72.  Hook-shaped  anhcdra  of  mag- 
netite and  hematite  [black]  pene- 
trating and  replacing  the  silicates,  es- 
pecially biotite  \hi]. 
*  195 


FIG.  75 

Hornblende    [ho]    rim  surrounding 
pyroxenes    \[>x]     (diopside     and    hy- 
persthene).     This  phenomenon  is  of- 
ten observed  in  magmatic  deposits. 
x  40 


Fig.  72 


Fig.  73 


Fig.  74 


PLATE  XIX 

PHOTOMICROGRAPHS  OF  SECTIONS  OF  ROCKS  AND  ORES  FROM  THE  ENCKI.S 
MINE,  PLUMAS  COUNTY,  CALIFORNIA 


FIG.  76 

Thin  section  of  norite-diorite 
showing  cliopside  [di]  with  rim  of 
hornblende  [ho].  Euhedral  crystals 
of  magnetite  [black]  within  the  diop- 
side,  and  anhedral  crystals  of  mag- 
netite and  hematite  [black]  along  its 
borders. 

x63 


FIG.  77 

Thin  section  of  norite-diorite 
showing  silicates  [light  gray  and 
white]  and  ore-minerals  [black], 
magnetite  and  hematite,  cut  by  vein- 
lets  of  chlorite  (shown  by  arrow), 
which  proves  that  the  alteration  is 
post-mineral. 


,FlG.    78 

Thin  section  of  ore-bearing  grano- 
diorite  containing  tourmaline  [dark- 
gray],  sericitized  feldspars  [light 
gray],  and  ore-minerals  [black], 
bornite  and  chalcocite.  The  feld- 
spars and  tourmaline  are  replaced  by 
the  ore-minerals  and  they  in  turn  by 
sericite  laths.  (One  is  indicated  by 
arrow  in  the  lower  right-hand  cor- 
ner.) This  section  shows  local  de- 
velopment of  sericite.  Most  speci- 
IIHMIS,  linwi'viT.  are  not  affected  by 
serialization. 

x  23 


FIG.  79 

Polished  section  of  the  ore  shown 
in  fig.  78.  Note  the  irregular  hook- 
shaped  bornite  [b]  with  narrow  rim 
of  supergene  chalcocite  [a"]0.  The 
bornite  surrounds  and  replaces  the 
silicates. 

x    184 


Fig.  78 


Fig.  79 


PLATE  XX 

PHOTOMICROGRAPHS  OF  POLISHED  SECTIONS  OF  ORES  FROM  THE  ENGEI.S 
MINE,    PLUMAS   COUNTY,    CALIFORNIA,   SHOWING   LOCAL 
REARRANGEMENT  AND  COPPER  ENRICHMENT  SUB- 
SEQUENT TO  THE  MAGMATIC  STAGE 


FIG.  80 

The  photograph  shows  the  follow- 
ing post-magmatic  alterations  of 
bornite  [b]  :  First  the  development 
of  the  so-called  graphic  intergrowHi 
of  bornite  [b]  and  chalcocite  [cc]lf 
and  later  the  development  of  rims  of 
chalcocite  [cc]0  around  the  margins 
of  the  bornite  areas.  The  first  gen- 
eration of  chalcocite  is  probably  hy- 
pogene,  the  second  generation  prob- 
ably supergene. 

x  125 


FIG.  81 

Anhedron  of  bornite  [b]  altering 
to  covellite  [cv]  and  chalcopyrite 
[c/>],  of  the  second  generation.  The 
latter  develops  along  crystallographic 
directions  of  the  bornite.  This  alter- 
ation is  probably  supergene. 
x  542 


FIG.  82 

An  area  of  bornite  [b]  has  been 
replaced  by  covellite  [«•]  and  chal- 
cocite [cc]^  and  later  all  of  these 
have  been  penetrated  by  chlorite  [ch] 
laths.  This  alteration  is  in  part 
along  crystallographic  directions  of 
the  bornite. 

x  142 


FIG.  83 

An   area   of   bornite    [b]    has   been 
replaced   by   chlorite    [ch]    laths    and 
by    quart/.    \q]    veinlets.      Chalcocite 
|  <v|.,    of    the    second    generation    has 
developed   along  the   margin    of   the 
bornite,     along     the  chlorite-bornite 
contacts,  and  along  veinlets. 
x  142 


Fig.  80 


Fig.  81 


Fig.  82 


Fig.  83 


WILLA:  D  j.  CLA; 

Hti!!.0   P 


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